rip out hiberfil.sys and disable system restore (use Acronis or Ghost etc and send your images elsewhere) and a stock install can be cut down to around 10gb. That's without your proggys of course but I have made do with a 32gb Xtreme since January and it still has *checks* 11gb free of 29gb. Just be sure to run disk cleanup every now and then to get rid of all the temp rubbish and system error reports ETC and you'll be fine. Huge proggies like Photoshop and the scratch I keep on a 500gb drive.
I just checked my own Win7 install and its just 13Gig and i fully installed it i think
so just going out to a store today to see if they have some but i rather have a bit more then 32Gig anyway so 64Gig is most likely what i am getting....