Well, not to put a damper on your plans, but if you're gonna be gaming a lot, you should probably look into AMD (YES, I HAD TO!!!).
For most games, a GeForce 6600 will suffice these days. You will need a bare minimum of 350W for a PSU with a 6600, and i suggest 400W or 450W (the more, the merrier).
If the mobo is ATX, then it will fit in any ATX case.
EDIT: Oh! And if you're gonna be gaming a lot, i suggest 1GB of RAM if you can pull it. 512MB will work OK, but 1GB will feel better.