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 HD4870 512MB, ATI650 HD Tuner, Phenom II 940 @3.35GHz, 4x2GB OCZ PC8500, ASUS M3A78-T, WDigital + 4x Seagate 7200.xx = 3TB+, Plextor DVD, PCP&C S75CF 750W, Enermax Full-Twr, 2xSony CRT (24" 16:10 + 21" 4:3), 5.1 Spkr, AT ATH-AD700 Phones, Cougar+FSSB, Saitek X52 Pro, Windows 7 HP 64b, Windows 7 RC 64b, Vista HP 64b