When I went from XP 32b to Vista 64b, some application software (expensive) would not work. And, there were driver issues for some hardware. I went through the pain and expense of getting things to work with Vista. I kept XP as a dual boot.
Since then, I upgraded CPU, MB, GPU.
So, my system works OK going from Vista to Windows 7 64b -- only one application needs an update. Games and older hardware actually seem MORE compatible with Windows 7 than Vista (where I can perceive a difference). Apparently, they got the drivers mostly right for Windows 7. All the drivers I needed were on the disk or downloaded automatically -- painless (at least in my case).
I've formatted my XP drive. Just don't need Windows XP for anything anymore. Really don't need Vista either but its still there.
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ATI Crossfire Sapphire 2x HD5870 - Eyefinity 5760x1200 24", 1xDell-U2410 H-IPS + 2xHP-ZR24w S-IPS, Phenom II X6 1100T@4.0GHz w/Swiftech Water Cooling, 16GB GSKILL PC3 1866@1333, ASUS Sabertooth 990FX, WDigital + 3x Seagate + Hitachi + 2x WD Ext = 10.0TB, Sony DVD, OCZ ZX 850W PSU, CoolerMaster HAF922, TM Warthog HOTAS, TM T-Flight Stick X, TM Cougar+FSSB & CH Pedals, Saitek X52 Pro & Pro Combat Pedals, TrackIR5 w/TC Pro, Windows 7 HP 64b