""SO it will be better to just do the room and not try to create a vocal booth.""
Well, that depends on how much work you want to do. If you want to have a nice sound proof booth for recording vocals then yeah, build a booth. If you built a booth you could make it a little smaller, which would save you money on absorbing foam and diffusors. If this space is all you have to record in and there are no other rooms then I'd suggest fixing up the whole room.
""About how many bass traps should I get. I think I can get them for about $25 apiece on marsmusic.com""
How many to get? It depends on how many corners you have. Where the ceiling meets the walls, you want them there. Where walls meet walls at 90 degree angles, you want them there. Bass trapping is something you always want to keep in place. The foam on the other parts of the walls and the diffusors should be installed so that you can move them around if you want to change the sound of you room. The reason bass trapping takes out the bassy room mode, and then you're going to translate the sound more true to tape/hard disk. You just want the room not to show up on the final recording... at least not in a bad way, reverb can be a good thing, but not boomy bass.
""Also, do you recommend any particular brand, and how many diffusors would be good.""
Some people might not agree with me, and I guess it's a matter of personal preference, but I do prefer Auralex. The only thing is, you're looking to only spend $400, and it just might not fit your budget. I havn't checked any other brands lately for prices though, maybe they're cheaper than everyone now.
How many diffusors would be good? Where ever you have parallel walls you want a couple across the walls from each other, but not mirrored. If you have one 3 feet from the corner, the one on the opposite wall can be 4 feet from the corner. Just so they're staggered. How many is up to your ears.
""After all this, will I still have to get those zigzag foam thingys or will the bass traps/diffusors suffice?""
Yeah, you will get the Wedgie-type foam. The bass traps just remove the bass modes of the room, and the diffusors cut down on slap echo. Once the slap echo is down you still want to put up some foam to absorb the reverb.
Just listen with your ears. Bass trapping may be all you want, at least you'll remove the bass mode. It's up to you.
Later,
-Brian