[KarmaKiller]

1 xIntel Core 2 Quad Q6600 G0 Stepping (2.4GHz 1066MHz) Socket 775 L2 8MB Cache (2x4MB (4MB per core pair) Retail Boxed Processor
Good choice. =)

1 x Sapphire HD 4870 512MB GDDR5 Dual DVI TV Out PCI-E Graphics Card
Great card for the price.
1 xASUS P5K-E/WIFI-AP AiLifestyle Series P35 Socket 775 Socket eSATA 8 channel Audio ATX Motherboard
I would check into the Gigabyte GA-P35-DS3L board. It's a bit cheaper, and essentially the same thing.

1 xCorsair 4GB Kit (2x2GB) DDR2 800MHz/PC2-6400 XMS2 Memory Non-ECC Unbuffered CL5(5-5-5-18) Heat Spreader Lifetime Warranty
As Ex said, go with 2x2Gb sticks
1 xAntec Nine Hundred 900 - Gaming Case with 200mm Top Fan - No PSU
Might check into the Coolermaster CM 690. Generally they are cheaper, and offer better air flow IMO. Great looking case too

1 xArctic Cooling AC-FRZ-7P Freezer 7 Pro Socket 775 Processor Cooler
That's a good cooler. Same thing I'm using on my Quad.

1 xCorsair 620W HX Modular PSU - ATX12V v2.2 APFC
Great choice. That is a really good PSU

1 xSamsung SpinPoint F1 HD103UJ 1TB Hard Drive SATAII *32MB Cache* - OEM
Also might check into Western Digital Caviar SE16 640Gb drive. They are almost as fast as Raptors are, and are under $100 here.

Looks like a nice build to me.