AFAIK, X300 GPU should be a 110nm 9600 (RV360), and X600 is a 130nm low-k 9600. Both are obviously PCIe-native versions of the 9600. Don't know if that X300 you have is 64-bit (9600SE) or 128-bit (9600), or what clocks it has.
Edit: IIRC, a plain X300 and X600 should have the same clocks as a 9600, the X600 Pro as a 9600P, and the X600XT has somewhat higher mem clocks as a 9600XT. I don't think there's a X300 Pro or XT, and I'm not even sure there are 128-bit X300s, though I *think* the X300 GPU has a 128-bit interface (thus the Dell carrying cards with a SE suffix).
But really, I think it's safe to say:
Q: How fast is it?
A: Slow.