A few more suggestions...
For RPGs I would say get Tales of Vesperia first, unless you have an aversion to anime-style graphics. I think they came up with better characters for that game than any of the previous Tales games, and it really makes it satisfying to play through (most of the older Tales games had characters that weren't compelling/got on my nerves). Don't forget it also has up to four player co-op.
After Vesperia? Lost Odyssey gets good reviews from many, though I couldn't get past the Final Fantasy parody-like beginning and FF: Spirits Within-like empty, emotionally bleak feeling that the world gave off. But I didn't get very far, so don't give my word much weight.
Last Remnant is a very good game with a few technical problems. Personally I haven't beaten it yet because I'm hoping the PC or PS3 version bring some improvements. If you do get Last Remnant, do the HD install before you start playing -- it makes a bigger difference on that game than anything else I've tried (some games actually get worse when you install them, so it's good to check online).
Fable II is a pretty awesome game and unique among rpgs, definitely worth trying out, and probably worth buying so you can take your time with it. The multiplayer is a bad joke, though.
Eternal Sonata is a really nice looking and consistently fun game, which you, which can't say about every RPG. Do you like Chopin? I'm a pianist, so the game is a marriage made in geek heaven for me. It also has co-op, for 2 people.
Two games it would be a crime to miss: Braid and Castle Crashers on Xbox Live. They only cost 10 or 15 bucks each, and they're probably my games of the year, in addition to LBP on the PS3. Braid you have to give about an hour of time before you judge it, it starts a bit slow for an arcade game. Castle Crashers is the best local co-op of the year, and the best beat-em-up in I don't know how long! Just make sure to play it with friends, all on the same system.
Seconding what others have said, Kameo and Crackdown are two games which can be gotten for cheap now, both of which probably qualify as hidden gems. Kameo actually has 2 player splitscreen which was added in a patch (that's why it doesn't mention it on the box) and Crackdown has online co-op.
Left4Dead is great fun, but if you have the choice go for the PC version, something about the framerate bothers me on the 360 version. An innovative game in a less obvious way.
Alic
