First and foremost, I'm very much a 'bang for your buck' kinda guy. I have a family I need to support, so I can't go around upgrading my computer all the time.
Currently I have:
I5-750 @ 4ghz (Stable and temps are fine, so might try to get to 4.3 in the future)
4GB's of RAM
I know for a fact I am getting a new graphics card, most likely a 670, possibly a 680 because I might be getting a deal from a friend.
I know the new GPU will provide a pretty noticeable boost to performance, but my question is, is it worth another 500~ dollars to get an 2500k and a new motherboard (eying the Sabertooth Z77) and perhaps upgrade to 8GB's of RAM? What kind of performance increase would I gain from getting these upgrades?
Your current setup is pretty decent and at that resolution I don't think your current components will hinder your new graphics card from greatly outperforming the 5870 and delivering smooth frame rates. My advice would be to ride this pc for at least another year though you could use some more ram if its currently limiting you. You're next major upgrade should probably be a Haswell processor, at the very least, depending on how good it turns out while carrying forward the GPU you're currently going to buy. Cheers!
WEll with your current set up I say you really wont ever really need to upgrade you CPU.. Intel CPUs are plenty powerful enough and will never limit your system so your fine with an i5 for the next 3 years at least! Upgrading a CPU would give you maybe a minor 3 - 8 fps increase depending on how much of an upgrade it is. I wouldn't want to waste another 500$ for an extra 5 fps! haha But yeah currently just stick with upgrading the GPU, you may want to add 4 more Gigs of ram in there if you are heavily multitasking and have a 64 bit OS otherwise 4 Gigs should be plenty. But I would heavily recommend to get a new Radeon from the 7000 Series for you GPU as if your not afraid to OC and push your card a bit the 7000 are very tolerable of Overclocking and you can easily maximize the price/performance. if your looking for a good 400 - 500$ card id grab a 7950 or a 7970. I am currently running a 7850 OC core 1000mhz Mem 1250mhz and I get around 50 fps in BF3 maxed and around 70 - 80 FPS in skyrim maxed with some enhancement mods.. So if you wan to go cheap get a 7850. Avoid the 7870 because the 7850 is just a toned down 7870 and you can easily get identical performance with a OC.