need help with precision workstation 420

ive had this computer since it was one of the best out there and it has served me very well although with all the programs i have it is starting to get too slow it's to the point where i cant even do my homework and listen to music and talk on msn all at once...so i figured id speed er up by adding another processor but when i put it in it powers up and everything just moniter stays blank and then nothing...if theres anyone out there that can help me it would be greatly appreciated