Good suggestions with a wide range of genres, pr2thej. Although for me I'd go with the new PoP over Assassin's Creed. If you like adult language, lol...Fallout 3 may be just the thing for you.
Titan Quest was a good single-player timesink, and the Gold version of it with the expansion pack is only $20 - heck of a good value. Hellgate: London is decent, too, considering you can pick up a used copy from Amazon for like $3 + $4 for shipping.
Mass Effect may also go in that list of my favorite games of all time...it has shooting elements like Fallout 3, but really can't be classified as a "shooter," so it shouldn't make you nauseous.
I'll second Fallout 3 and Mass Effect. Fallout 3 is (imho) the better of those two games. Be careful with GTA IV... make sure your PC is up to spec (before you buy it) if you're going to try to run it!