There are benchmarks that show graphics performance is better on amd64 than the standard P4. Also the price per performance is alot higher on AMD market. I use an AMD64 as well with ultra 160 controller and it does rock. I believe the cpu to memory bandwidth is higher on the AMD64 systems then P4's as well.
But the big performance gains come when you use the AMD64 FX chips , which might as well be an opteron.
Think there's some benchmarks here:
http://www.aceshardware.com/
There are definitly some here that prove the point that AMD64 is worth the time.
http://www.thejemreport.com/modules.php?op=modload&name=News&file=article&sid=117
As far as the database apps , I will be able to tell you fairly soon what the performace is like as we have a new opteron system we'll be running Oracle on