depends on what your need are, your video card is fine for EQ.
the gtx460 and ATI 5850 are both in the 200-260 price range and great cards for the price. going any higher is like paying a premium for the small increase you will see.
not sure how much you know about overclocking but most core 2 duo chips overclock well. you could overclock your chip to around 3ghz. overclocking is not for everyone, it takes little greater understanding of how PC's work to do it but, you should have no issue finding a guild to assist you.
http://hardforum.com/showthread.php?t=1198647 here is a database of C2D overclocks reaching 3ghz with a E6400 should be pretty easy.
http://hardforum.com/showthread.php?t=1138241right now i would consider a video card updated and overclocking your cpu. then save your money for a full new system. intel has moved on new sockets so while you could upgrade to like a 3ghz chip yours will overclock to that for free. just have to understand the risks. i run cpu/gpu overclocked 24/7 just keep everything cool dont overvolt more then few percent and you dont have to worry about killing hardware.
