The Geneforge games do have an overeaching story arc but unless the price has dropped dramatically for the earlier games i would just go for the latest installment, it certainly won't spoil your enjoyment not having played the previous titles.
With Avernum it comes down to how much of that type of gameplay is enough for you. I purchased the last one and enjoyed it but don't think i will be diving back into another one anytime soon.

I like a good story. Is the story in Avernum thin?
Thanks, I might look into buying Geneforge V this week.

It's on sale at Gamersgate, in case you missed it.

I find Geneforge to be better overall, it has a deep multi-part story and animations as well as really good combat. Avernum doesn't have animations and I've found it to be quite jarring. The stories connect from game to game, but not as much as Geneforge. Both are great though, to be honest though my preference has been Geneforge.

Much like Eschalon, you don't need to have completed Geneforge 4 to enjoy and understand what's going on in Geneforge 5. I would say though that it is worth picking up and playing Geneforge 4 at least, simply because there are a lot of plot elements, events, places, and characters that connect them (I won't give away any spoilers!). A hardcore fan would recommend you even go back to the beginning to GF1-3, though I wouldn't go that far, myself.
Avernum is much the same way: I've played IV - VI and they all fit so well together. Again, there are a lot of people you meet, places you travel to, and events that referred to in the games that refer to past games that really enhance your experience. The story of the Archmage Solberg is one example - tracing the way his character changes from IV to VI is quite tragic, and you don't get the full sense of this unless you've played past games in the series.
One of the hallmarks of Vogel's games is that story of both Avernum and Geneforge are huge. Vogel's an expert in creating a vast, expansive world with heaps upon heaps of things to see, do, and interact with. In fact, the story aspect of Vogel's games is pretty much one of their main selling points.
Whether you'll like it though is a different story - if you haven't tried a demo of GF5 or A6, you'll see that Vogel and Spiderweb pretty much poured all of their effort and talent into storyline, and plot development, at the expense of graphics and sound. IMHO though, any RPG fan owes it to him/herself to play through A4 or A5.

Thanks for the comments. It's too bad Gamersgate doesn't have the entire Geneforge saga on sale. Maybe I'll get Geneforge 5 and Avernum V first.
Another question: does it severely affect my playing experience if I play the games in a different order? I'm just thinking that, in the event I do finish G5 and AV, and Gamersgate does get the older games into its catalogue, I would like to try them too. But, as far as my gaming experience is concerned, I don't usually play series in backwards order, since I'm quite worried the "impact" will not be there anymore. Let's just say that I am of a mind to play through the entire series when I do have the time - is this backwards/sequential jumping going to have an adverse effect on my experience as a whole?

I would really like to play these games, but I do not understand why the developer has to put DRM on the games. It is a bit disheartening as I would have already bought both series if they were DRM free. Eschalon Book I and II are DRM free by-the-way and are very good games.

According to Vogel's blog, there's no DRM on their games.

There are certainly twists within the Geneforge games which are not revealed if you play them out of order, apart from some of the major plot developments such as the emergance of the splinter factions.
I purchased G1 and then went onto 4&5, if your wallet can stand it i would recommend a dual purchase of the latter two Geneforge games...at least try the demo of Rebellion.
Whilst there is an ongoing storyline within the Avernum universe from the one game i have played and an extended playthrough of another demo i think you can play them in any order you like without ruining anything for yourself.

I bought Geneforge 5 and Avernum 5 in the GamersGate sale.
Tried them both for about an hour each, and I think I prefer Geneforge but it's a bit early to tell.
I do feel like I've missed out on a lot of background stuff, but I don't think it is hindering me too much.
Gamersgate versions require a serial key, but that seems to be all the drm. The Mac versions appear completely drm-free, and I think they're drm free if you buy direct from the spiderweb site?

If you buy direct from Spiderweb, you get a demo, which is actually the full game and they give you a key to unlock it with the purchase. That's the extent of the DRM.

FWIW, I played Avernum out of order (started with 5, then went to 4, then up to 6) and I didn't really lose out on the development of the plot. Then again, I'm guy who doesn't mind reading spoilers on games or books (except in some rare cases), so I suppose it depends on how you feel about that.
If you buy from Spiderweb directly (which is what I've done), you won't get any DRM on any of their games; there's just a simple serial number which is intended to deter casual piracy. If you buy from any other retailer (e.g. MacGameStore, or Big Fish Games) you run the risk of getting the game wrapped in whatever DRM system they use.
Myself, I've always preferred Avernum; I never got fully used to the different party dynamics or Geneforge, where it's just you and the army of shaped monstrosities you spawn, as opposed to Avernum's more traditional approach. I also liked the fact that Avernum (at least, A4 and A5) have two sides which are a lot more clear cut in terms of who's really good and who's really bad. I always had this unsettled feeling when I played Geneforge, that the only reason why I allied with one side wasn't because I believed in their ideals but because they were only slightly less (or more?) evil than the other guys.

Hmmm I guess I'll just hold on until this Sunday. Meanwhile, I'm going to email Jeff and check if he's going to get the rest of the games to Gamersgate. Seeing as to how it's best to play the games in order, I think I shall adhere to that lol.

I think you can still enjoy the series a great deal even if you don't play the games in the "right" order. i bought Geneforge 5 from GG and liked it enough to buy the rest of the series from Spiderweb Software (where the games are DRM free by the way, if that's a concern).
I found it very interesting to see the events that shaped the story, it didn't spoil much imho.
Keep in mind, though, that I'm in the habit of beginning books in the middle or the end, and spoilers don't bother me much, so your mileage may vary.
If you're unsure and want to really check out the games, there are big demos (they cover about 20% of the full game each) for each game available from spiderweb's website.