System looks god, but I just want to clear some things up:
1. Why are you going from an already bought 480W PSU, to a $39 420W PSU??
2. I know your hard drive isn't the fastest in the world with only a 2Mb cache, but you might want to consider getting another one exactly like it, and put them in a RAID function as your motherboard is capable of it? It would be cheaper and you get an additional 40Gb compared to that 80Gb Seagate.
3. Your buying another 6600GT without buying an SLi capable motherboard? Surely keeping your very good overclocked 6600GT would be way more beneficial??
4. Why don't you just spend that little bit more (i.e. the savings from not buying a less powerful PSU) on your motherboard to get an SLi and put the new 6600GT in there to almost double your performance in the graphics section?
5. All the items you put down in your list come to $611.49 without shipping and tax, so you may need to ammend some stuff in there.
Hope all that helps!
<center>Athlon 64 X2 Dual Core 4200+ 512KB l Gigabyte GA-K8N-XP Sli Motherboard l XFX GeForce 7800 GTX 256Mb (450/1250) l OCZ 2Gb (2 x 1Gb) Dual Channel Platinum Series DDR400 l Maxtor Diamondmax 250Gb SATA 7200rpm 16Mb Cache l Enermax 600W Noisetaker l NEC ND-3540A Black Dual Layer DVD+/-RW l Sony 3.5\" FDD l Black Coolermaster Aerogate 2 Fan Controller l Extreme Gaming Spider Case l Viewsonic VX924 19\" TFT 4ms Response Time l Logitek MX518 l Saitek Eclipse l Windows XP Professional Service Pack 2</center>