My local Fry's Electronic still has it's Q6600 on sale: http://shop1.outpost.com/product/5101696

You can find better memory prices at NewEgg, even 4 GB modules for that price.

If you do happen to go for 4 GB modules, get a 64-bit edition of Vista with SP1.

If you're not SLI'ing, I strongly suggest checking out a P35 based chipset motherboard.

Now that I noticed that you're using the Ultra X3 600W power supply, I would suggest replacing that immediately. That particular power supply is only really good for ~450W. Check out [H] video on their load test and see that it shuts down when under heavy load here:

http://www.youtube.com/watch?v=9s1hY0W0hPk

8600 GTS OCed > 2600 XT

However, I'd recommend checking out the 8800 GS:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150275