PSU refers to Power Supply Unit. You should see the rating on the back of it when you look behind your computer. Just look at the watts for it and that will give you your rating. if it says 650w that means it is a 650 watt rated PSU. The higher the wattage rating the better because the graphics cards hog a lot of power.

Well if you're after games on here, you MAY not need anything newer. Whilst the video chip you have isn't really that fancy, it's still more modern than most of the games would need. Things like Freespace or Descent should work fine.
I'd recommend trying one of the games on here you're interested in, it may well run perfectly and if not, it's not an expensive loss.

If you have a Celeron 2.66 GHz processor then you probably have either a socket 478 board, or if it's 775 a very early one. In either case it's highly unlikely the board has the necessary PCIe slot for modern cards, and I'm also guessing your PSU is around 300 W at best. Without replacing your PSU, mobo, processor, and RAM (basically building a whole new computer), your best upgrade path would probably be to track down a 7600 for an AGP slot, although because these are old cards you typically have to pay around $100-150 for them (not exactly worthwhile for such dated hardware). You might want to just consider scrapping your whole system and putting together a new budget system for around $500-600.

Not really, you can get socket 775 celerons as low as 1.6GHz so it could be anything.
highpines, roughly how old is your computer?

Lots of processor lines went under "Celeron". The Celeron Core and Celeron Duel-Core lines were LGA 775 and had clock speeds down to 1.6 GHz, but not up to 2.66 GHz. 2.66 GHz is almost certainly a Celeron D, a series which was available on both 478 and early 775 sockets.

I have a 2.7 ghz Celeron and it's certainly not a Celeron D. It's in a HP that I bought in 2002. I upgraded from integrated graphics and since it only had PCI slots (not even AGP) and a 250 watt PSU, I was severely limited to what I could do. I ended up buying a used ATI 256 mb 9250 for only $10! It plays 2002 - 2004 games ok (I did upgrade to 1 gb RAM from 256 mb) but is really good for anything earlier. I've never attempted to play anything new. Celerons aren't fast enough for serious gaming.
Take my word for it, unless you just want to use it for GOG or casual games, don't even bother upgrading it. Celerons stink for gaming.

What stinks for gaming are pci cards.
I've used AGP since 2001. PCI-Express since 2005 (When I got a pentium D and a 7600gt).

That must be from the generation before the Celeron D, probably a Northwood; the possibility of it being one of those had slipped my mind.

Looks like I bought this computer in 05' and here is some of the system info........."Processor x86 Family 15 Model 2 Stepping 9 GenuineIntel ~2700 Mhz" ...This afternoon I will take a look inside see what slots are open if any. Nothing on the back about power supply. I used to build my computers back in the 90's so if anything I can try to catch catch back up alittle.....I'l let you know, Thanx

You know I was re-thinking this a little while ago and your absolutely right. PCI cards for games really are bad. But being a Celron doesn't help it one bit. They are definitely slower than a similar P4.

If you can run sisoft sandra, that'll let you know heaps of info on your motherboard that'll help us make recommendations

well
I ordered today brand new Sapphire 4850.
4870 is a bit too expensive for my taste and that card was only 272 bucks with delivery. (that's 172 american)
http://www.skycomp.com.au/product.aspx?id=126049

I recently bought a MSI Nvidia 9600GT for about $74.00. IT was well worth the short money it cost.

this
i got the 9600gt from newegg a couple of months ago
along with my 2 gigs ram and 2.2 ghz athlon x2 the thing can run crysis on high(!)
amazing value here, if you are upgrading just pay the extra $20 or whatever, you'll pat yourself on the back in a couple years