Definitely get the fastest dual core you can and max out your memory. I use Premiere and it is hungry for both, but for now I think you will open many more issues with 2x2. With XP4200 and up you can do real time editing if the rest of the system is up to snuff. Anybody had any experience with 2x2? I do hi end audio editing with Adobe, vinyl restoration and such. I'd be interested to hear.... :roll:
2 Single Core model Opterons running at 2.6GHz each would be more cost effective and faster / core.
If you have highly parallelized applications then the 4 cores would be faster overall however each core by itself would be slower so all clock sensitive applications wouldn't run quite as fast as they would on the 2.6GHz single cores.
A lot would depend on your OS' scheduling policy too and other factors.
Here are some disk benchmarks on FC3 i386 and FC3 x86_64 on a Dual Opteron with RAID5: