What is an equivelent PCI-E card to my AGP one?

Hi all, I have a ATI Radeon 9800 Pro 128mb.

Just wondering if I were to buy a PCI-E system, what card would provide a similar performance?

I guess it's just out of curiosity!

Hope someone can help :) .

Thanks

Ash

a 6600 DDR2 perhaps? or an X600XT