I'd recommend a Pentium 4 600 (get the 65nm 'Cedar Mill' though) or Pentium D 900 (65nm 'Presler') to be honest.
The software you are running won't actually benefit from a 4-way system, and likely not benefit from Pentium 4 XE/ Pentium X either (as they have 2 cores, each with HT, but the image editing software will only use 2 most likely).
Yes, they may run in over 8 threads, however they only run 2
isolated threads. They could benefit slightly from the Pentium X, esp if multitasking while editing, but not what I'd class as a significant enough improvement to justify the cost.
It'll benefit from a dual-core / 2-way system though.
Disappointing..... perhaps, but it may save you some cash. 8)
My Opteron 270 isn't that great at image editing, but it works (for example):
That one was made by me yesterday afternoon, but the original is far higher resolution.
The Pentium D would be a far nicer system for your needs, with Intel Matrix RAID support (RAID-0 and RAID-1 on the same 2 drives, giving benefits of both, but low cost of only 2 x 160 GB+ HDDs 8) )
http://www.intel.com/products/processor/pentium_D/index.htm
Perhaps a Core Duo (they perform well, even in image editing, they are lower clocked, cooler, but do more work per clock cycle):
http://www.intel.com/products/processor/coreduo/index.htm
Opterons, even dual-core ones, just don't run image and video editing software at the same pace as the Pentium D IMHO. Owning a quad-core (over 2 x dual core) Opteron 270's I feel able to say that with confidence.
In fact it would be better at image editing than my rig, at a far lower cost, unless you've got image editing software that can run in 4 x
isolated threads. Sure mine would be better at multi tasking video editing + image editing at once though.
http://www.microsoft.com/windows/windowsmedia/forpros/encoder/default.mspx
The Pentium D / Core Duo have higher raw SSE performance for what you pay, and alot of the software is compiled for the Pentium 4/D and ends up performing slower on Athlon 64 / Opterons, even if synthetic benchmarks (each compiled for given microarchitecture) indicate otherwise.