First they are many other MB that will do the job and cost less. C-net is running a course right now on how to build your own PC and one on building a MCE PC.
Since nic cards are cheep just get one that linux will see.
check out for prices.
BTX MB and PSU are just starting to come out, but a 300W PSU for this P4 processor is lacking, should be a 400W or better.
In this price range you migth want to look at AMD64 processors also. John