Just upgraded an XP Pro to Win7 64. Did a clean install, left 75G for the system.
After installing MS Office 2010 and really not that many other apps (Photoshop CS2, Quickbooks Pro, Firefox, Skype, Dropbox, AdAware free, print driver, etc.) the 75GB system partition is about 40% full. And I haven't installed Acronis TI Home 2011 yet. 75G turned out to be OK, but if I did it again I would leave 100G for the system.
Also, because this PC had XP on it before, and the 2 drives were already formatted into one big 465GB single NTFS partition (RAID 0), when I installed Win7 Pro it did not create that 100GB "hidden" boot partition. All my boot files are on System (C:\)