If you got a 64 bit processor get XP 64 as i use xp 32 and 64 and even with 2 gb memory i can render bigger scenes in poser 7 even mem hogger under 64.
Good news is looks like Microsoft going get windows 7 out next year which is good news indeed, not for vista users though, as they will have an obsolete operating system.
Whatever microsoft try and do with vista they cant fix the faults unless they rewrite the whole operating system, and windows 7 will be 64 bit only.