At work, we just migrated a database server from a Sun Fire V240 to a Sun X4100. This makes it the first AMD64 system we've put into production, and the performance advantage is staggering. I could post the benchmarks and various statistics, but I believe the following graphs paint a far more interesting and convincing argument for the price/performance benefit of Sun's AMD64 offerings...
Before (V240) CPU Utilisation
After (X4100) CPU Utilisation
All told, I'm impressed. The X4100 is ripping through queries at a phenomenal rate and is barely breaking a sweat. The V240 on the other hand was clearly struggling and was maxing out at 100% load. True, it's not a true like-for-like comparison, as it's pretty much impossible to do that across different systems and different architectures. But take a look at the price levels of these two systems - the V240 came in at around £7,500 for dual 1.5Ghz UltraSPARC IIIi processors, whereas for £4,800 you can get the X4100 with dual dual-core AMD 285 processors clocked at 2.6Ghz. Frankly, it's no contest. The only thing you don't get with the X4100 is another couple of disks which is no big deal as we've hooked it up to our SAN. However, even if you want to go for the X4200 which has room inside for 4 internal disks, you'd still only end up paying £5,100.