If you're slots are pci-e 2.0, then no, there will be no bottleneck whatsoever. However, pci-e 1.1 at 8x will have a slight impact on frmaes. Maybe 5-10%. However, you will gain so much performance from running two cards that it won't matter. Make sure your CPU can keep up.
I would recommend an intel core CPU at about 3 ghz to elevate a CPU bottleneck.
Post your system config...
Goodluck!