Assume you hacked the boot.ini and all that other stuff first, right? I attempted an XP dual-boot when I got on the 7 beta, but it didn't work; XP boot got hosed. Never went to the trouble of fixing it. I spend very little time in Windows on this machine (though that may change if I get a couple games for Christmas).
I've got 7/64 running on my media center PC. I've damned and praised it elsewhere. On the whole Ubuntu handles privilege escalation much more painlessly, and also was much easier to set my Wii remotes up in. (Windows's built-in Bluetooth stack blows, BlueSoleil costs $30 and still didn't consistently listen for my remotes at boot, and GlovePIE, while a very neat program, is buggy, won't close properly when I try to shut down, and isn't exactly sophisticated programming; I'm considering whipping up my own Java code for WiiUseJ.) However, running a Wii remote in XBMC under Ubuntu/64 doesn't work that damn well; you push a direction and it's like roulette guessing at where the cursor will land. I'm sure it's something about the polling delay being based in a 32-bit number which I could probably fix trivially in source, but I can't get it to compile from source because it's a bitch to compile anything 32-bit on 64-bit Ubuntu.
I've got Windows 7/32 on the spare computer in my back room, and it's quite zippy on a P4 with 1GB RAM and an old Matrox video card, even with Aero shit turned on. That said, it runs about the same speed as Xubuntu did on the same computer with 256MB RAM. (The only reason I switched was that, while Comic Collector Pro runs under WINE, it throws up a whole lot of very annoying error messages. That's mainly what I use that computer for, is comic inventory, as it's in the room with my comics.)
On my main desktop...KDE 4 is finally fully-featured enough to work as my main window environment (though the taskbar is completely fucking atrocious and violates every usability guideline in the book; it doesn't even have simple drag-and-drop functionality -- I guess it's fine if you like it exactly the way it comes by default, but if you want to change it in any way it's going to be a huge, time-consuming pain in the ass. But at least it's actually doable now.), but it never seemed quite as fast as 3 did to me. After some mysterious lockups I switched back to 3, but I can't confirm that 4 was what caused the lockups. (I thought for sure it was causing the shit a few weeks ago where my mouse click would register in a completely different window than the one I was in, which pretty much made me throw up my hands and quit using my main computer for about a week, but it turns out that was X itself. It's fixed now.)