If you're building a system now you should go with a Core2Duo ... even just get the cheapest one (E6300). Also get the dual channel ram to give the two processors maximum flexibility.
P4s were never great anyway and are now pretty much obsolete (the core2 was built from the P3s design). A trader will have more than just tradestation running and even if TS is only single threaded TS will run on 1 processor while other things will run on the other.
WSAM, if its using a lot of ram something has to move the data around the ram ... and thats the processor. The e6300 is not expensive.
I built a system at Xmas with an e6600, 2G of dual 800MHz ram and all the normal stuff including widescreen monitors and had paid for it a few days later. Nothing is processor bound or ram bound now.
If your processor is lightly loaded by all means just improve what you've got. Be careful about mixing slow hard drives with fast ones because, when on the same cable, they'll be limited to the slower speed.