1- The FX and Extreme Edition versions are generally obscenely priced for being a few percent faster. Although... if your productivity depends on your computer's render speed, then it might actually be worth buying.

Although in that case, a dual core dual opteron might start making sense??

2- As far as benchmarks go:
There's rendertest.veg results, although no Pentium D results for it. (Can someone please post their results?)

There's benchmarks for the Main Concept MPEG2 encoder on various hardware sites. However... all the results vary for supposedly the same benchmark.
Intel has a small edge there, with the dual cores about the same speed as single cores.
For AMD, the dual cores are faster than the single cores at MPEG2 encoding.

3- If you're looking for a system to string you along, maybe look at the Dell Inspiron 9100 (dual core Pentium, 2.8ghzX2). There's various ways to get a deal on one of those, and it'll be cheaper than assembling a computer yourself.

That system should be very comparable to a dual core AMD or whatever's fastest. Heck... get two and you have a network render machine as well as a backup computer.
For about $2500... you can get:
2 Dell Inspiron 9100s
2 X 20" LCD monitors
$500 is for hard drives, DVD burner (~$60), more RAM ($110 or so for 1GB), KVM switch (~$100??) [may not need?], 10/100 ethernet switch (~$40), dual DVI video card *You have to install this stuff yourself. You may need a little more than $500, it's just a guestimate.
here's the link, it may not necessarily be the best deal you can get