The R200 IGP is pretty close to an X300SE in terms of potency, and a refurb vanilla 6600 for my box was an unbelievable upgrade. It was like night and day. Went from 800x600 low settings to 1024x768 medium-low settings at higher frame rates. GW no longer uses a directx8 renderer.
Double the shader an texture ops, almost triple the memory bandwidth. It's a big step up.
The 7300GS is slightly less potent than a 6600. About 7% less GPU power across the board, and 20% less memory bandwidth. But still a giant leap up from a X300SE.
You make the call.