i don't know about the motherboard, but i'd recommend the pentium d processor. the d has two proper processors, the ht makes one processor core appear as two processors, but the overall performance boost with ht is a lot lower. while the pentium d runs at a lower speed (2.6ghz), it does more work per cycle than the pentium 4. also, the larger cache will help with server workloads.
for similar reasons, you should get centrino processors instead of pentium 4 or pentium 4-m processors for laptops -- larger cache and more work per cycle. the new core duo and core 2 duo processors are based on similar technology to the centrino.
if you can wait a few months, i'd recommend getting the desktop core 2 duo processors. the pentium d is being phased out.
for comparison purposes, games like quake 4 on my 128mb nvidia 6200/sempron 2600+ processor (with an unhappily low amount of 128kb L2 cache) run at 30-50 fps under linux (windowed mode, 1024x768, medium graphics).
unrelated: the cache is why certain processors are better for servers. for example, this
server from ibm. 1.9mb L2 cache, and 34mb L3. they have other models with 70+mb L3 cache. i believe some of the hppa processors (from hp, used for hp-ux) had up to 3mb L1 and 64mb L2 cache. the itanium 2 processors have up to 24mb L3.