The following article shows that the Q6600 is a very capable performer when compared to other processors:http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3641&p=7
I like Leon's advice to replace the motherboard and overclock the Q6600 to boost the performance. This will only cost you about $120.
If you really want to get another 30 percent boost in performance, and are willing to spend $390, then a i750 CPU ($150 at Microcenter), a Gigabyte P55-UD3R motherboard ($140), and two sticks of 2GB RAM ($100) is a good option. The i750 / P55 computer actually outperforms the i920 / X58 system for gaming because the i750 processor incorporates the PCIe bus on the chip, thus eliminating the bandwidth bottleneck associated with the northbridge chip required for the i920 processor.