Being more of a console gaming guru, I'm not knowledgeable on what most of these games are. I've already picked up Fallout, Fallout 2, and Descent + Descent 2, and I'm probably going to pick up Fallout Tactics and Descent 3, but what else do you recommend? Please give reasons. :)
Thanks in advance.

Original War....
... Because it's pure RTS/RPG extasy

I do love me a good RTS. Command & Conquer... <3

It is a lot different from C'n'C, but it is also near the best RTS I have ever played

I think, there should definitely be a demo service, because in all honesty, the site is called "Good Old Games" so they should be, logically all good.
But I think MDK is worth a look as well as 1nsane.

I got Giants and it's loads of fun. A bit annoying sometimes because of the difficulty, but the humorous storyline is totally worth it.

jagged alliance 2: because it is what fallout tactics wanted to be but wasn't really. squad based, turn based tactical strategy game. hire your mercenaries, equip your mercenaries, and then use them to kill (just about) everything in the room. good fun.
freespace 2: because its amazing how well this game stands up after 9 years. everything about it is well thought out and polished, from the backstory archives, the control scheme, the graphics, even the sound and voice-over work is excellent.
i've heard good things about perimeter, but i'm not an RTS fan, so i couldn't say for myself.

I'm a bit of a console gamer myself, so I'd suggest Original War (if you like RTSes, and great stories), and Kingpin perhaps.
Giants:CK was a console game too, so that'd be good.

As has been said, Giants:CK would be a good choice.
But I second Freespace 2. Your Star Fox avatar tells me you'd like it. ;)
It's simply stunning.

Outside of the games you already bought, the best games on the service are Original War and Freespace 1&2.
Original War is a very unique RTS games. And the Freespace series are simply the best space combat sims ever.

"Hey man, you got a dollar?"
"No, I ain't got no dollar."
"Come back when you got a dollar."
"@#$% YOU!"
"@#$% YOU MAN!" <starts charging at you>
Gotta love Kingpin. :P
Oh yeah, and get Giants: Citizen Kabuto. Awesome game that one is.