The Q6600 (Go stepping Version) runs at a stock 2.4ghz. With a very cheap cooler I've overclocked to 3ghz with very little increase in temperature. They use less voltage you see. This gives me comparable performance to a Quad extreme for 1/3 of the price. There are loads of websites with comparison reviews.
I used to have a 4400 dual core AMD, this is twice as fast in real terms. I would recommend getting this over a dual core as some of the newer games (Crysis and COD4 being the first) are optimised for quads. I can run Crysis at a good res with nearly everything on high at about 60fps, not bad. There isn't really that much difference at the moment between QUAD and Core Duo.
The new Penryn based systems run up to 1666mhz FSB, where as mine is 1066 FSB. It makes a lot of difference, they are still Intel 775 motherboards so all the current CPU's will work. I have heard reports of people overclocking the Penryn to 2ghz FSB with good results, amazing.
And I wouldn't recommend SLI to anyone. For double your money, you get 30% performance increase (provided the game supports it) I had an SLI system a while ago, and it didn't do what it promised. If you can afford it go with an 8800GT or above. The difference in cost to a 9 series Nvidia GPU isn't worth it either, and the dual 9900 is a rip off. They huge too, and eat a ridiculous amount of power.
It basically comes down to what you can afford really, I would keep that nice case though. lol