The Q6600 is not avialable any more
. It's old. It's slower than the AMDs mentioned. You don't need a 5870 for a triple core. You don't even need a video card for it.
The only Intel CPU that's a reasonable priced upgrade is the i5-750 at $195. After that, you're looking at an i7-920 build, which runs a good $200-300 more than an AMD build.