Currently I have a Q6600 clocked at 3.0 (It no longer makes 3.6, over the years it got pretty burnt out). I was thinking about buying a new quad, but with the new i-core series in mind, should I just build a whole new machine? At what point does the quad core processor line become obsolete?
Unless you get a "really" good deal on a s775 based CPU, I'd consider upgrading to a more recent CPU. It all depends on your budget too. If your running with DDR2 you could upgrade to an AM3 (AMD) based mobo that takes DDR2, but since DDR3 is sooo cheap right now that wouldn't make too much sense. Your PSU/Case/DVD/HD are possibly still good to transfer over to the new machine, so you can keep costs down, unless you want to start with a fresh build.
What's the rest of your system? If you already have a good 8GB of 1066 RAM and you're sure it's your CPU and not your motherboard that's limiting your overclock, then it may make sense to get a 45nm quad core off eBay and stick with that for now till you save up to upgrade the whole thing.