this is by no means scientific. but the question is based on an ordered stack of priorities.
putting aside the theoretical "the more ram the better" and assuming that the only thing you are using your computer for is resolume ave:
the first priority being:
1. how much ram does resolume+os need... i can not see any benefit of having more 3 gig of ram on a designated res ave pc. I can not get my total memory commitment to go higher than 1 gig with avenue not getting much higher than 350mb. I have 46 process (xp pro) going on and I have a good 2 gig free ram sitting on top of everything, making me feel comfortable. (i am not multiprocessing, only res ave is running during a show)
Unless you are gonna juggle some RAM disks to play your media off of, i do not see where 12 gig of ram will benefit at all.
2. so if ram is not the reason to run 64bit then the second priority is if the software is 32bit or 64bit.
as of now, resolume is 32bit so until 64bit programs become more standard, there isn't any major positive reason to go 64bit for a res ave machine.
so that helps decided 64bit or 32bit.
3. so when it comes to selecting a 32bit os for res ave... winxp pro is the best solution as it is the smallest resource hog between XP, windows7, Vista, or OSX.
plus there are some tried and true tweaks to really slim down xp:
http://www.tweakhound.com/xp/xptweaks/supertweaks1.htm
(I am unsure if there are is any OS coding in win7 that is geared to take advantage of the i7 processors, 4+gigs of RAM or RAID0 drives. I have read that there are some unique optimizations for SSD drives in 7...)
all that said, if you design on the SAME system that you use resolume, there are reasons to go 64 bit win7 when it is available. I design on a separate machine (using After Effects CS3) and on THAT system i am running Vista business 64bit with 8gig of ram.(intel 3ghz quadcore extreme)
While I have been satisfied with my performance and stability when vista64 has 8gig of ram at its disposal, I will being upgrading to win7 after it comes out.
maybe others have more hands-on practical input about the other OS options and their possible 64bit benefits...