the DP35DP and a QX6600 will give you a good midrange system, 4 proc allow some headroom, i'd use memory matching the FSB (1066)
8 GB will take you reasonable far, though desire always comes with options ;-) to add a second slave in case really needed might be a good advice - just to add to the switch: it doesn't need to be managed (unmanaged but capable of layer2 switching is enough)
important note regarding XP vs. VISTA ... with VISTA the TCP/IP network stack has changed, so the decision IMO depends only what your master computer will run. VISTA-to-VISTA and XP-to-XP gives better performance on the network than VISTA-to-XP or XP-to-VISTA
also if deciding to XP make sure in advance all devices (including the onboard ones) have drivers for XP64 ... this can become an issue since XP is on its end of life ... basically XP is less hungry on ressources
christian
and remember: only a CRAY can run an endless loop in just three seconds.