I'd never bothered with 64bit before, it was always just that little bit too geeky and misunderstood. Luckily with the advent of Windows 7 i thought "hell, why not?" and decided to give it a go.
Obviously the big upside to this is the support of larger memory, so I decided on a new set of Crucial Ballistix Tracers which showed up today (pushing me up from a respectable 2gb to a more beefy 4gb), threw them in and WOW.
Loading times for levels etc have dropped almost by half, Framerates are up in most of my games, Windows and the various programs on it now open almost instantly with barely a rotating egg timer in sight.
So, if anyone is looking for a quick boost to your PC for a reasonable price I can't stress enough. 64bit and the memory to take advantage of it is probably your best bet.