I'm not sure you wasted your money on the quad as future games may use them more efficiently plus you said you use multiple apps. As stated, currently games respond better to faster cores rather than 2 vs 4.
You should aim for 3.2 and over to get the best frames out of a 4870.
3.6-3.8 is very achievable on an aftermarket air-cooler with an E6750, you'd need water-cooling to do that with a quad.
Just installed a loop on my brothers QX6700 and it really needs that watercooling to get past 3.2. Huge differences in temperature between the E6750 at a similiar OC.
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Xeon 3350/E6750| DFI X38 LT| 2 x 4870|74gb Raptor
Thermalrock Dragon Case (sprayed black)
CPU loop: MCR 220 int.|Apogee GT| MCP655
GPU loop: MCR 320|Swiftech Res.|2x EK FC blocks| MCP655.
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