HD4870 still has reasonable performance. Your CPU could overclock a little but all in all you got a balanced system.
To do a gradual upgrade with a quad and reuse the HD4870 is fine. Your CPU can handle a more powerful card, you can reduce bottleneck by overclocking to 3+GHz.
I personally would start though planning a new build either with LGA1155 or bulldozer which is supposed to release the next quarter. If budget is an issue then the HD4870 still has life left in it and can be upgraded at a later date.
Thank you for the reply. It's OC'ed now from 2.4 to 2.8, but I honestly don't even know if I did it right. If I remember correctly I just upped the multiplier a little I think, and under load it currently gets to about 56C. From what I've heard, C2D's can get to about 65C safely. Is that correct? I just don't know if I feel comfortable OC more, I'm afraid I'll damage something. Is bulldozer going to use the AM3 socket? Or a whole new one? Maybe I should just OC the CPU a bit more for now, buy a better GPU, and then get a new mobo and CPU once bulldozer hits.