Mass Effect (RPG heaven)
Crackdown (sandbox paradise)
Dead Rising (Sandbox nightmare, but in a good way)
Bioshock (included just to include it)
Halo 3 (but you need an HDD to get it's full potential)
Uno (nevermind, you already have that compilation disc)
Aegis wing (it's free!)
WarTech (super Japanese)
Yaris (just kidding! this one sucks ass!)

dead rising
dead rising
dead rising
viva pintata
crackdown

yes someone else who thinks wireless gaming is a waste 

A lot of these posts have already hit on the obvious games...

I would say check out the XBLA, I've spend quite a bit of money there even though I have access to both the PSN as well. I would recommend the following:

Alien Hominid HD - The Gamecube game with HD graphics. Pretty tough but a fun side-scrolling shooter.
Bionic Commando Rearmed - Get this on PSN or XBLA same for both, just get it for the controller you prefer
Braid - Already recommended but I'll second it again.
Castle Crashers - Beat-em up goodness. A ton of fun if you got people to play local with you.
Castlevania SOTN - You've probably already played it but still worth throwing out there
Geometry Wars 2 - Skip the first game and go for this full blown sequel. Very good leaderboard integration and lots of different modes to keep you occupied. I still go back and play it even with all the new games out. Very addictive.
Ikaruga - Probably played this before but if not it's a great SHMUP
Pac-man: CE - Yeah it's pac-man but seriously at least try the demo before you discount it completely. I've had a blast with this game.
Rez: HD - Again another classic but still fun to go back and play today. HD graphics + surround sound = awesome experience.

For my 360 that I just bought, myself, I got the following. There are only two games I want for the system that are currently available & not included here - Infinite Undiscovery and Tales of Vesperia, both of which were too expensive on eBay as of yet (compared to these titles), and will be purchased sometime in the (possibly) near future.

Bioshock	Blue Dragon	Call of Duty 4: Modern Warfare	Gears of War	Grand Theft Auto IV
Half-Life 2: The Orange Box	Kameo: Elements of Power	Lost Odyssey	Lost Planet: Extreme Condition	Mass Effect
Project Gotham Racing 4	Soul Calibur IV

People!! lets not forget he has a good computer. So stop mentioning Mass effect! It runs bad on the xbox360 compare to a PC, so if i was going to recommend Mass Effect, I'll do it for the PC.

OT: I'll say get Blue Dragon, Tales of Vesperia, Infinite Undiscovery, Kameo, and Dead Rising.

I own all 3 consoles as well, but the only games i have on my 360 are RPGs and Dead or Alive 4. Still better than my wii lol. I only have Galaxy on it. For some reason, i find paying full price for wii games a rip off lol.

get mass effect and blue dragon. i thought blue dragon would suck, but so far its pretty good.

XBLA all the way.

Braid, Castle crashers, Geometry wars 2, Catan, N+, Ikaruga, rez hd, soul calibre etc etc XBLA is really the best thing about the 360. Cheap fun games ftw.

In regards to retail games, everyones said them all really. Make sure you pick up fable 2, TLR, Star Ocean etc when they get released. The 360 really has a huge library so your not going to have any trouble finding good to great games to play. Plus since its older then the competition you'll be able to find lots of really cheap games aswell (such as kameo).

You guys are suggesting a lot more XBLA games than are going to fit on that 256mb memory card. Guess I should look for a hard drive pretty soon.

Thanks though.  Honestly you guys have been a huge help, and suggested a lot of stuff I hadn't thought of.

Naz, I don't know your philosophy on reselling equipment but I would look into picking up a used 20gb hard drive.  Target had new ones on clearance a while back for around $30 but I doubt you'll find any there now.  Used they still go for around that price on ebay.  You could even sell that memory card if you wanted to recoup some of that cash.  I don't think $30 will break most people nowadays but gas is on the rise... It makes the most sense if you want to expand your library and play a few games online (you do get a 1month free gold trial and who knows what the future will bring).