The 800x600 resolution will run your 480p games just fine, but 720p games cannont be effectively shown in High Definition on a 800x600 projector.
That's not a huge deal now, but when Xbox 2nd gen comes out, you'll want it to shine with your projector. Might as well get a higher res.
As for the wall. . . I understand you're renting, so you may not be able to sand the wall flat and paint it.
I believe what Icky was saying, but I'm an A/V guy and a science geek. The truth is, if you shine a light on a rouned surface (even if it's a bunch of tiny rounded bumps), the light gets scattered, instead of being reflected accurately. It's like throwing a baseball at a basketball. The light gets bounced every which way, instead of giving a consisten picture to everyone from many angles. Flat walls are best. But bumpy ones will do.
As for the screen. . . A screen would be best, but if you can't get a screen, A smooth wall with flat paint will be the next best thing. "Flat" means it's not glossy. Now, some will argue about the best color of flat paint to buy. Some say white, while others say off-white, and some even say light gray. But flat paint is definitely what you want on the surface if you can't have a screen.
Really, a bunch of the guys here can attest that you'll get a decent image using many setups, and several setups will do, but I'm just trying to briefly let you know what's going to give you the highest quality image possible.
As for trapezoidal (keystone) adjustment, definitely get one with keystone ability. This will allow you to mount your projector at any respective angle, and adjust for the non-perpendicular alignment to the surface you are projecting onto. Most projectors sold today have that ability.
Also keep in mind that most projectors have a VGA (or equivalent) input to allow you to hook up a pc or laptop. This would be great while you're away at college because you could do your homework on the big-screen in your room, just for the fun of it!
