PCI E 2.0 is backwards compatible to 1.1, and 1.1 is mainly forwards compatible with 2.0, which is great.
if you have a 4870X2 for example on a P35 (pci e 1.1 x16) i'm pretty certain it would be bottlenecked. but a card like the 8800GT, or even a low end card like the 4650 or 9600GSO which are rated at 2.0 specs would not be bottlenecked by a 1.1 slot, or if so, bottlenecked by an unnoticeable amount.
but seriously. if you're buying a 1.1 card to put in a 2.0 slot... you're a little behind the times lol
Can`t find out either but the PCI-E 2.0 spec was n`t issued until Jan 07, so I`d guess your board is either 1.0 or 1.1. Either way unless you go for a X2 style card the loss in graphic performance would be more theoretical than noticeable.