If you have the Win 7 or can get licenses I much prefer Win7 over Vista. 32bit or 64bit is depending more on how much memory you can or need to use. Some good graphics setups need the 64bit as they get a little limited with only 3Gb or so addressable memory. Similarly, database work and similar progs can use huge chunks of memory to speed access/sorting etc. Memory is getting cheaper and most systems only need a max of 6Gb on X58 Intel 3 channel @ max of 1600Mhz. Faster shows little benefit unless you want ultimate benching or bragging rights. On 2 channel AMD systems, if the bios can let you address the full 4 Gb that should be more than enough for daily needs and even the full 3Gb available on most other systems is enough really.
I am upgrading a few systems direct from XP as after trying Vista I found it just too bogged down whereas I've had Win 7 running quite happily on an old Centrino laptop with 1Gb and 1400Mhz. Vista just wasn't viable. Family pack licenses, coming soon, will give you 3 systems at a reasonable (for Micro$) price but i got a couple during the (very short) offer period and I've been running 5 systems on Win7 beta and the later code since early days with few problems. RTM code should be pretty decent I reckon and XP, apart from XP mode and VM stuff for legacy stuff, will get consigned to the drawer for me. Just wish I still had access to some Corp license packs tho'