You didn't say what you are upgrading FROM, so I'm assuming you already have a socket 775 system. If you have anything other than that, then it is a simple choice, buy a complete upgrade package including an i7.
I have an E8400 and if I was looking for just a cpu upgrade I'd probably just go for one of the faster quad core socket 775s, on the theory that a poorly behaved single thread is less likely to pause my computer if I have an extra 2 cores. I wouldn't do that particular upgrade for higher game framerates however. If I was looking for game performance rather than just a bump to system responsiveness, I would certainly go for an i7 cpu, no question about it. It is a generational leap ahead and while some games will run "just as ok" on the older cpu line, do you really plan on playing just those games for the next few years?
Even civ 5 on my computer would benefit from a couple of upgrades. It pauses during drive accesses even though I have a WDC caviar black, one of the better 7200 rpm drives. An SSD just might fix that, but for the price of a good SSD I could do a complete system refresh with a new i7 mobo, cpu, and ram. So I'm keeping my pennies for when I need it.
If you are happy with your system and just want a faster cpu, then get a fast dual or quad core upgrade and be done with it. But if you are looking for noticeable speedups across the board and not just in AH, then you might not be happy unless you jump to the current generation of cpus.