I currently have an i3 540 @ 3.06 GHz and with stock cooler, and am wondering what I should do as an upgrade. I also have 8GB of ram and an EVGA GTX 480 with a corsair 600w powersupply. Would it be worth it for games to upgrade my cpu to an i5 760 or an i7 870? I know those are older generation processors but I only have an 1156 motherboard and don't want to have to upgrade that as well.
If I don't upgrade the processor then I would probably get an SSD as I currently have a 320gb WD hardrive.
I would try selling your old cpu first for around $50, and wait until you get the newer one before removing it; I've seen i5 750's in the $120 price range on craigslist, but remember you'll need a separate video card with that cpu.
For most games the 540 is still okay. For games that heavily favour quad core CPUs, like larger Battlefield 3 multiplayer games, Grand Theft Auto IV and the like, it might be worth upgrading to a quad. Future games may become more heavily threaded, so if you want to keep this system for a while it might be worth upgrading to a quad core.
The i5 760 is probably your best choice for that platform. Literally the only difference between i5 7xx and i7 8xx is hyperthreading, which virtually no games actually use, and slightly higher stock clockspeeds on the i7s (aside from the i7 860 which is clocked the same as the i5 760). The i7 875k does have an unlocked multiplier, but that probably isn't worth spending an extra $100 just for easier overclocking.
An SSD would probably provide the best overall boost to your system, if you aren't playing CPU heavy games. It will make Windows load a good deal faster, and make your system snappier overall. Won't help game performance though, aside from shorter loading times for games that you do install on the SSD.