Uh what
You can't just say "An AMD dual core" or "an intel dual core". A Pentium D (dual core) at 4Ghz will be outperformed by a wolfdale core2 duo (dual core) that runs at 2Ghz.
Similarly a Phenom X2 (not released yet, though) at 3Ghz will outperform an Athlon X2 that runs at 3Ghz.
It all comes down to
IPC.
Your Q6660 is a (65nm) quad core that runs at 2.4Ghz. A 65nm Phenom X4 9950 which runs at 2.6Ghz will perform about the same as it.
The Phenom II is coming out soon, which is both runs faster and has a higher IPC, and will be the best CPU for the price in my opinion. Intel has a new architecture out called Nehalem, and these are the "Core i7" cpus, but they're super expensive, plus you'll also need a new motherboard, memory and so on to use them.
A Q6600 should be fine till like 2010 at least, CPUs aren't the performance bottleneck in games anymore, your video card is.
If I was building new I'd go for AMD and get a socket AM3+ setup with a Phenom II in it. But since you're on intel and socket 775 you'd have to go for a new 45nm quad core to get better performance. Just keep your Q6600 and overclock it =o