My last computer was a dual boot XP 32 bit and Vista 64 bit and later upgraded Vista with 7 64 bit.
Initially I spent about 95% of the time with XP, but that turned round to about 99% on 7 for the last year I had it.
Windows 7 64 is the way to go for a number of reasons, but DX10 and 4GB + ram are two important ones.
One very important point to note with an XP and Vista/7 dual boot set up though; When you boot XP it will delete all your system restore points on your Vista/7 partition !
Microsoft know about this (their help function tells you this
) but for some inexplicable reason they have never fixed this bug (or is it a feature)
.
So if you install something onto your 7 partition or if you get an automatic update then I would recommend you boot back into 7 to ensure everything is ok. If you go into XP first then you wouldn't have any restore points to roll back to if there was a bad install.