Well, I'll be dual booting and Linux will be my primary OS, so I want the Windows partition to be as small as possible w/o being too small as to induce severe fragmentation. I intend to allocate 60GB or so to Windows, so having half of it all gone before I even start installing any games is a problem (it'll probably just be SupCom, WiC if I can find the disk, MTW2, and/or Spore when it comes out; although I might be satisfied just leaving them only on the desktop) .
Right now after install VS2005 and OpenOffice, among some other small stuff, my C:\ has 32GB in use (or 22 according to WinDirStat

). Of this space, 16.8GB is taken up by Windows (according to Windows Explorer), 8GB of which are in c:\windows\winsxs . I believe prior to getting a 2nd hard drive, my desktop windows partition was only 50GB (also running Vista x64). Programs take up ~5GB, the largest of which is Visual Studio at 2.5 .
I've left Aero on since my graphics chipset is fairly decent (Quadro 140m), and it does seem to make various GUI stuff appear smoother. Performance problems seem to lie in disk I/O and memory, though probably wouldn't be as bad since all I've been doing so far is installing/uninstalling crap. The biggest concern is disk space though.
One more thing- how do I change the color in the menus for legacy apps? I utterly hate the light blue that appears in the menu for default Firefox and Pidgin (fixed the former w/ a theme), but I haven't used Vista enough other than games until now for me to bother doing anything about it.