Some of the best computers around at the moment are multi socket 6 core AMD cpu's, for price you can't beat them in the server market. The new supercomputer list is also very pro AMD, 4 of the top 5 fastest supercomps in the world are AMD right now, the 5th is Xeon based, but its power comes from using over 5k 4870 cards in the supercomputer so its still basically an AMD comp.
Its not really about CPU power, but how well you can use them.
Personally, I wouldn't be looking to upgrade a Q6600 if it can get anywhere near 3.5Ghz you'll see almost no real world benefit going to any other quad, especially for games. The next next gen stuff, Bulldozer and whatever follows Nehalem might be the time to upgrade, not necessarily straight away either but we'll probably see some of the more Crysis type games out in two years start to really push a quad core and beyond properly.
It depends what you do, considering you have a 3870x2, I'm assuming you do game and thats your primary focus for needing power? A gpu upgrade won't require a CPU upgrade to be worthwhile, a decently clocked Q6600 is more than capable of running any current game with any current GPU setup maxed out, they are/were great great value quads for their time. Upgrade your gpu, maybe up to 8gigs mem, maybe a decent SSD and leave the cpu/mobo for a couple years.
We're very close to moving to a whole new type of system set up tbh, where CPU's are largely Interger whores, with reduced FP power(faster than now, but less FPU power in ratio to the Interger power) then as intergrated gpu's are moved on die, and can be used as a dedicated FPU within the CPU we'll see the new "CPU's" FPU power go through the roof. THe first proper Bulldozers won't have intergrated gpu's, I doubt Intel's high end follow up to Nehalem will have a gpu on die either, because they'll be too big, the following die shrunk versions should bring gpu's on die at which point software/gaming/industry could change quite drastically.
In terms of the server market AMD are planning to really undercut Intel, well the early rumours are they are removing the price premium for higher socket count versions of their chips. IE a single socket 3Ghz 6 core AMD server chip costs less than a 2 socket version of the same chip, which costs less than a 4 socket version, and so on and so on. They are apparently moving to a , 3Ghz server cpu can work in almost any setup and will have a single cost no matter how many you use. It would seem over the next two years we really have some radical change in overall system power/composition and architecture and until that happens and software maybe starts to change to utilise the extra power/cores nothing inbetween will really offer your average gamer/home user much real world performance increase.
I wanted to borrow one of the tigerton setups to set a 3D Marks 06 WR but we can't find a board with x16 PCI-e heh.