Is there a reason not to go for 64bit win7?

[Raumarik]

I suppose it depends on what games you play
The only one I found ran faster on win7 was DCS Black shark , when I benchmarked my
games under both O.S.'s

Which of the games you tested on both O.S.'s ran faster under win 7 64bit ?

L4D, Saints Row 2, NHL09, Sim3 - not actually played that many games but there's a noticable improvement.

One I've still to try that i'm wary of is GTA4, I'd be shocked if it was faster in Win7.

Not benchmarking anything, just general feel/smoothness has improved although that could be loading times and general lack of garbage on the OS install.