ATX should be fine, so long as it has the studs for a mini ATX motherboard, which I believe it should.
You should check ahead of time for the minimum requirements of the games you want to run, of course. That's probably going to be the most daunting part - since video card makers don't follow a linear model number system - ie a Foo580 is actually a better card then a Foo4000.
When I jumped to Win7 I found that all the old XP games I had worked fine in Win7.
The big thing was that I had one game where I had to set the game folder permissions manually to be so anyone could do anything in the folder.
Seems XP is automatically that way, whereas Win7 defaults to much tighter security that can block a program from doing things like writing to its own config files.
Games were: flight simulator 2004, trains 2012, and some stuff off Steam Portal, Portal 2, Fallout New Vegas, Noir. Steam stuff may have self updated... Who knows?
My $0.02, YMMV.
