So, for around $500 you get a dual channel Opteron motherboard and an Opteron 144 running at 1.8 gHz. Slower than an Athlon 64 3000+ speed-wise, but it will probably outperform it in real world with the dual channel memory, or at least come close to it. PLUS, it wil have a longer life expectancy as far as upgrades go, probably.
<font color=blue>When all else fails, throw your computer out the window!!!</font color=blue>
Have you checked pricewatch lately? You can get an Asus board for FX that will also take an Opterion for little over $200. Even if you did get a server board, you would only put one CPU on it. On the memory, it's more money and hard to find fast modules, but they are also more stable.
Also, check the prices on your Athlon 64 boards, you should be able to do at leat $100 better.