It'll bottleneck some, but it's in many conditions, variable to the game or the application. The gtx460 after all, is about equivalent to a gtx 280. You are fine for now, however, if you go sli, you really need a new mobo and processor. And as far as I remember, the chipset on your motherboard doesn't allow for core unlocking. But, if you still want to go amd, there are a lot of good deals lately. As a matter of fact, newegg was blowing out a few CPUs like the phenom II x3 which almost certainly unlock to quads for 69 bucks. If you could score a cheap mobo, that would be a good setup.
FYI, the new intel chips are coming out Q1 of next year, so prices on i7's, i5's, and i3's should begin to go down. I'd overclock your processor if you have a decent cooler for it, when faced with a bottleneck from a graphics card, you see more performance gains from overclocking a bit.
Overclocking to 3Ghz only brought my temperature up 5 degrees when I maxed out the fans due to the fact that I have 6 fans going through this thing, and my new case has 3 200mm fans and 1 or 2 120mms, so it should keep things pretty cool. Also the case supports water cooling, which will be something I'll look into. If I've got the cash, I still might be able to swing a new mobo and processor as well as new RAM seeing as DDR2 is hard to come by unless you buy a shit motherboard.