The memory bandwidth (GPU<->VRAM) has nothing to do with the PCI-E bandwidth (GPU<->System). I highly doubt that the PCI-E 1.1 bus will be much, if any, of a bottleneck for another generation or so, since the 9800GX2 can more or less keep up with the GTX 280, so the results from this test: http://www.tomshardware.com/reviews/pci-express-2-0,1915-12.html should still be valid, especially for anything slower than a 9800GX2 (such as the 4870).
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