That's a tough call. An upgraded GPU can be moved into your next rig; not so with a new CPU. Have you tried overclocking your E4600 a bit more, say to 3.2GHz-3.4GHz? Try that and see how much performance improves, if any. If it doesn't, then a faster GPU will help more. I suspect it will though, but consider how long you will keep this rig before spending a lot on a CPU for a dead socket.