I'm sorry, I didn't read the original post carefully.
For single core, get an AMD chip if you are sieving. Get a Pentium 4 based chip if you are running the SoB client (Northwood vs. Prescott doesn't matter much). The faster the chip, the better (L2 cache size is not critical as long as its at least 512KB). Avoid Pentium M and Core (as opposed to Core 2) chips. If you do both SoB and gaming or other CPU bound task you have a dilemma, AMD chips are generally faster at most other tasks whereas SoB prefers the Pentium 4 Netburst architecture.
On the digression I started, Core 2 has two FPUs! A 2.4 GHz Core 2 outperforms a 3.8 GHz P4. See
http://mersenne.org/bench.htm
Oh, and no matter what you decide make sure last week's steep price cuts are reflected in the chip or system you purchase!!!