About 6 weeks ago I went from:
3ghz P4
3gb mem
256m nvidia
XP

To:

Q6700 quad core
8gb mem
640m nvidia
Vista Ultimate 64bit

I could not be happier. The 64 bit lets me use all 8gb of memory and I'm able to do bucket sizes of 1024 now. My rendering is much faster than the old machine. I felt no real need to do the latest/greatest for Windows, but really wanted the 64bit and MS had done a lot of updating for 64bit between XP and Vista. I've had no driver or software errors yet. Only drawback to date are all the security features in Vista I had to turn off to get anything done, and I cannot load my Dragon Naturally Speaking on it because it cannot run in 64bit (yet).

I heard the next version of poser will be 64bit capable too so would expect renering to improve then again...