I recently went from a i7 870 to a i5 2500k, and I haven't noticed much difference. I had the 870 running @ 4.0ghz, and my i5 running at 4.5ghz, and in gaming there were little gains to be seen, maybe 2-3 fps depending on the game, maybe nothing depending on the game. I think the most notable was Crysis 2, and that was like 4-5 fps difference, but who wants to play that game anyway? j/k, i'm sure there are a few.
If you don't have a budget, then yes, spend the extra $300-400 on a new mobo, and i5.
If you are on a budget, you would be better off taking that money and getting a better GPU. I don't know which GPU you had in mind, but if you were eyeing up a gtx560ti, or hd6870, I would say spend the extra money on a gtx580, or hd6970, and you will be much happier. That's just my 2 cents.
Also, I'm using a Crucial C300, which is a pretty fast sata III drive, as an OS drive, and I haven't noticed it being any faster on my p67 board, than on my p55 board. It's always just been fast, and made the OS feel incredibly snappy. I guess if you are planning on using the SSD to write large amounts of data at a time you would benefit from the increased throughput of sata III, but for everyday OS use, I'm not sure it's really necessary, just my opinion.
Also, you may want to check out this thread
. ekg84 did a nice comparison of an i3 550 vs a 2600k using an OC'd unlocked hd6950. You can see there is little difference between the i3 550, and 2600k in gaming using a single higher end GPU. This further proves my point about the GPU. An i7 860 + high end ($400-600) would give you much better results than an i5 2500k + midrange ($200-300) card. Again, I don't know your budget, or what GPU you had in mind, but to me 20-30 fps, is better than 2-5 fps.Edited by anubis1127 - 10/7/11 at 11:33pm