There is really is nothing wrong with socket 754. They actually perform just as well as 939 -- the 1 or 2 percent gain from "dual channel" memory is already factored into the CPU's speed ratings.
The only consideration is that 754 upgrades will be more limited, since dual core CPUs are only offered in 939 (and a new socket coming out in a few months).
If the price difference is major and you just want single core CPU anyway, then I'd do 754. Plus they have some low power consumption CPUs for 754 only (turion), would be nice to save a little power and cut down on noise.
Yeah, I think 754 is a good bang for the buck right now, considering it's "old technology". I invested in "dual channel" memory, and never saw a noticeable improvement in performance. I like 754 because I can use a 400w PS (I have 2 extras here), instead of needing a 500w'er. I guess my thoughts are to save a few bucks on this system, and get a monitor I can nail to my wall. LOL