$950 to upgrade? Not worth it at this point in time, especially considering that your major bottleneck will be the video card for a relatively long time (in computer 'years'). Instead, you'd be best off waiting a couple of years and then spending ~$250-$300 on a much much better processor than if you spend the $950 on a processor upgrade now.
I would not pay $950 for that upgrade. Stick with the Q6600. It is a very good proc and will last you a while imo. That system will be just fine how it is with 4gb of memory, 2.4ghz quad and an 8800GTX imo. Save that money for when you later upgrade your vid. Hope this helps!