Is PCIe really worth the cash?

[Ventricle]

I want to get a new gfx pretty soon, at the moment i have a 9600XT 128meg and i was wondering whether investing in a new mobo to run a 6600GT PCIe is worth the cash. Would the performace be vastly different if i simply stuck with my current AGP mobo and bought a 9800pro over getting new hardware? Also, does anyone know a good site that lists the raw specs of the 9800pro and 6600Gt PCIe side by side in bench marks?