What frequency would one have to run an E8400 to unbottleneck a 4870. I have heard 3.6 and 3.8 GHz thrown around but am unsure as to how reliable those numbers are considering I haven't a clue myself.
core 2 duo at 2.4 is more then enough already as demonstrated by several review sites by running real life gaming tests. Going up to 3.0 ghz core 2 will give you a dozen or two extra fps depending on the game but any higher clocks then that will only start showing even more diminishing returns.