Welcome to SPCR, Hawkeye1
You might want to see if you can swing a Socket 939 Athlon 64 -- these will be the standard in the future. Though the Socket 754 are certainly less money. Apparently, AMD's production has hit stride, and they have shipped more Opterons in the first quarter than they did all last year...so prices may come down some.
I have built 3 Athlon 64 machines; one 2800+ and two 3000+. My favorite motherboard (so far, out of four that I tried), is the AOpen K89 MAX, A close second, is the Asus K8V Deluxe. The K89 MAX has almost everything you could want, including undervolting, fan control -- but it does not support Cool N' Quiet in the current BIOS. But if you Fold (for SPCR, natch), then this is not critical.
The Asus, does support CN'Q, but it does not undervolt, and the fan control is not as good.
RAM latency and compatibility is an issue: single sided sticks are much better, apparently, but I have not been able to tell which 512MB sticks are single sided -- if there is
such a thing. Get RAM with 2.5-3-3-4 or better -- I found that I was unable to force better timings on Mushkin PC4000 3-4-4-8 RAM, even though it should
have been able to do it.
Having said that, I had great luck with Mushkin's least expensive PC-3200:
and their Level 1 Dual Pack:
I did use the 2-2-2 Special stuff on the Asus, and it was fine, but it is a lot more $$ than the Level One stuff, and is barely any better.
Whatever RAM you get, you might have to push the RAM voltage *up* in order to get it to work right.
You will not be able to use the SLK-800A HS -- this is a different socket altogether. I used the Zalman 7000A, and the Alpha, and on one, I used the stock HSF (undervolted quite a bit) and while it wasn't as quiet as I had hoped, I was forced to use it because the backplate was glued onto the ECS motherboard that I was using.
A 300watt PS should be fine -- that's all I ever used in the three machines. BTW, I love the Matrox P650 but the ASRock and the Asus BIOS's often conflict with Matrox cards -- you have to disable their logo in order not to garble the POST and BIOS screens!
One the ASRock K8S8X, I could not *ever* get it to work!