Same question as Don Coyote. I do a lot of editing of large static graphic images & composites using a P3/800 and have no speed problems using a Matrox G450 Dualhead with 2 matching 19" screens - plays DVDs movies nicely too.
If you're doing a lot of games or movie editing or serious image rendering you may be happier, but don't upgrade just for the sake of it. A big monitor may be a better bet.
I'm replacing young Tom's old P2 with an Athlon XP 2000+ on an MSI 745 Ultra AMD ATX 266MHz+DDR ATA133 + 256M 266MHz DDRAM + a GE Force 4 MX 420 and this represents a reasonable cost/performance compromise for a games machine.