One idea I came up with was to set up a dual boot capability of XP Pro and the 64 bit windows 7, at least until I was comfortable with the new OS. Though I'm not really guru enough to pull that off without significant research, but free 7 for a year would be cool, could track down a cheap copy later.
I was running XP 64-bit and decided to install Windows 7 on a separate hard drive in case I wanted to go back. I haven't yet. The performance difference is too great. I'm getting 30 FPS in dalaran at the moment with my 5 online at 1680x1050 resolution with lowest graphics settings, which is a big jump from 12-18 or so I was getting before. The lag in loading textures in Dalaran is also better (less time) than it was in XP. Oh, and I use 1 WoW folder, period, on a 7200 RPM SATA drive, sound on all of them. My personal opinion at this point is moving to Windows 7 from a previous one is probably a better upgrade than an SSD drive (loading times seem much faster on the same hardware), but that's just based on my experience and I haven't used an SSD drive myself.