The Microsoft.Com Operations blog just posted about their experiences moving from x86 to x64, most notably the increase in performance they achieved by doing this.
The numbers are compelling:
To give you a quick comparison:
X86 ASP req/sec 7.85, Response time 244ms
X86 ISAPI req/sec 110.85, Response time 248ms
X86 Static req/sec 41.9, Response time 135ms
X86 Static (cached) req/sec 47.11 Response time 1ms
X64 ASP req/sec 7.41, Response time 53ms
X64 ISAPI req/sec 125.43, Response time 18ms
X64 Static req/sec 31.01, Response time 3ms
X64 Static (cached) req/sec 54.51 Response time 1ms
And this is performance gained just by running a 32 bit application (ASP.Net 1.1) on x64, via the WOW64 emulation layer (WOW indeed!)
Worth a read.