Well
Well, first I need to know if you are running on AGP or PCI. A X800 Pro is selling for arround $200 on Newegg.com (which is a great site), and a 9600xt is around $100.
X800:
Core Clock: 400MHz.
Memory Clock: 980MHz.
Memory Size: 256MB.
Max Resolution: 2048x1536 at 85Hz.
Memory Type: GDDR3.
Interface: PCI Express x16.
9600xt:
Core Clock: 500MHz.
Memory Clock: 600MHz.
Memory Size: 128MB.
Memory Type: DDR.
Interface: AGP.
In short, the X800 has a much faster memory clock, more memory, a faster interface, and a much faster type of memory. The difference in the core clock doesn't really matter, as there is a different typs of Memory, and a new engine. I would deffinately upgrade if I was you. Hope this helps.