The majority of the bottlenecks come from dual-card configurations, and ATI tends to do better than Nvidia when dealing with non-high end CPUs according to the Tomshardware charts.
Overclock the E6300 a little more and you should be good to go. (If you wanted to OC all the way, the E5200/5300 can both do around 3.8GHz, so the E6300 can do 4Ghz without too much effort)
Otherwise, the charts stated that the E6300 @ stock would bottleneck a 4890 or greater. So your current configuration would reduce most bottlenecks for your GPU.
4890 < GTX285 < 5850