Which version of Win 7 32bit or 64,  both work well.  With the 32bit version of Win 7 you are limited to 4 gigs of ram on the 64bit it will use all the ram you can afford.  The same goes for Daz Studio the 32bit version has a ram limitation but the optitex dynamic cloth plugin will only work in 32bit Studio.  The 64bit version of Studio advanced works great in Win 7 64bit, but you can run 32bit studio in Win7 64bit.  Studio 64bit and Win7 64bit is not faster it just allows those of us with more than 4 gigs of ram to use the extra memory to better advantage; ie I can load larger scenes with more models without the program running out of ram. If you are more into gaming and ocasional render or two and have less than 4 gigs of ram stay with Win 7 32bit.  If you are into 3D in a big way and have more than 4gig of ram go Win 7 64bit.

Hope this helps