I've been testing around a bit, but still not quite sure what I should go for.
At the moment - my specs:

Q6600 oc'd to 3.4 ghz
8800 GTX 512 mb ~10% oc'd
4 gigs of OCZ Ram
150 gig 10.000 raptor
300 gig 7600 wd

2x 24" LCD @ 1920x1200

I currently have vista x64 installed on the raptor drive, and XP 32 installed on the 7600 wd drive.

On vista I run dual view, But I often find my game 'locking' up for like 1 second, which is very annoying, and framerates aren't as good as using Span on windows XP 32
However span on XP is a tad bit annoying and it only recognises 3.25 gigs of my RAM (which is supposed to be 4, but limited due to 32 bit)

I'm planning on starting a 5-box again, if framerates are decent on 1 comp (main screen takes full 24", other 4 all a little part of the other 24")
Though I have 2 other comps of which 1 can run 2 wows and the other can run 1.

What would you do? XP, Vista, 32 bit, 64 bit? span or dual view?