Definitely the GPU, both will need upgrades at some point, and with bulldozer around the corner, it is worth it to wait and see, perhaps the 2500k will even drop in price as a response to bulldozer, or bulldozer might have a chip at the 2500k price level that beats the 2500k. Besides, to upgrade the CPU its not just a cpu, its a motherboard too, so the less involved choice is GPU.
dont bother with either one.. I was debating the same thing got a 955be myself. Just wait till bulldozer hits the market that should kill the sandy bridge series but like everything else its all hype. the 2500 and 2600k will drop in sept and for the record the bf3 demo played at e3 was a 2600k with a gtx 580. I heard BD hits the street sept 19 but yet again i heard the 26th. Then youve still got to wait for bf which is oct 25 at that time all the pricing will be down well maybe not bulldozer but the 580 will drop for sure. if your looking for a quick fix just oc your cpu buy a 30-50 dollar heatsink youll be happy with the results.