I'm with the others - fill 'er up with RAM. Buying the Tualatin (512K L2 cache) is a good move if you're going with a Dell box. Penguin Computing sells Athlon solutions, check this out:
http://www.penguincomputing.com/store/configurator.php
With 2x1.2HZ Athlon XPs, 256MB RAM and 4 18GB 10K SCSI drives and a network card, you're out $3800 and change. Their memory prices are high, so you could buy three 256MB DDRs from Crucial (registered, ECC) for about $45/each and still hit 1GB RAM on your $4K budget. I think MuseaTech's bought from Penguin before and are happy but don't take my word for it, you might want to email Talli Somekh.
Solving general performance issues will certainly be of very high priority once we get a solid release out the door...
