I'd also like to throw in the original Mass Effect since OG took my original suggestion :P
Go for the Ultimate Edition of Dragon Age: Origins though if you can. As it has all the DLC on the bonus disc.
That should go without saying (insert winking emoticon).
Beyond that, if you have access to a copy of the original Xbox version of Knights of the Old Republic... I consider that to be my favorite game of all-time (making it IMHO the best RPG). It's a little buggy on the 360, but extremely playable. After that,
FFVII is one of the greatest games ever, but that's not happening on an Xbox.
Yeah, I have it (KotOR) for PC too. I lent it to a friend though. Right now, I'm running an i7 quad... When I did install KotOR, I got a message saying the game was incompatible with my hardware or the resolution wasn't supported or something like that.
I never really tried to play it before I lent it out. Any idea what's up with that message?
I really liked FFVIII too, but it never sucked me in the way VII did.