At stock clocks, the AMD is the better bang for the buck. But if you do plan on overclocking, the the Intel solution becomes more attractive as most Q6600's can easilly hit 3.0Ghz.
As for future upgrades, the AMD would probably be better as the AM2+ is a newer chipset, where as the Intel 775 is starting to age. However, like one person mentioned, the new AMD CPU's were supposed to work in the older AM2 mobo's, but we now know they don't.
Just check out TomsHardware for their CPU charts and find an app that takes advantage of the 4 cores and see how the two CPU's compare. But also don't rule out the E6x50 series as they can be overclocked nicely and have a high stock speed out of the box. While only 2 cores, might still be as fast if not faster than the quad AMD.
As for future upgrades, the AMD would probably be better as the AM2+ is a newer chipset, where as the Intel 775 is starting to age. However, like one person mentioned, the new AMD CPU's were supposed to work in the older AM2 mobo's, but we now know they don't.
Just check out TomsHardware for their CPU charts and find an app that takes advantage of the 4 cores and see how the two CPU's compare. But also don't rule out the E6x50 series as they can be overclocked nicely and have a high stock speed out of the box. While only 2 cores, might still be as fast if not faster than the quad AMD.
