The Dedicated Graphics Power Supply can be smaller, because the GTX 460 uses a maximum of 160W. It would be good, however, if you would SLI the 460s, then it will need 320W max. It is just as a side note. The upgrade from a 8800 GT to a GTX 460 is not a smart one, IMO. You can still play games, can't you? Anyway, it's up to you.
GTX 460 is compatible with X38 chipset. There should be no issues.
You should be able to use a GTX 460 on your current power supply:
Thank you for the information and your perspective. My reasoning is that although, yes, I can play games, I've had to turn down graphics detail to moderate settings to run games like COD Modern Warfare smoothly.
With a game like COD Black Ops, I have to turn everything all the way down or off to run the game sort of smoothly at a resolution of 1920 x 1200.
According to the Tom's Graphics Card Hierarchy chart
an upgrade of at least 3 tiers higher on the chart would probably result in a noticable difference in performance. It is 4 tiers from the 8800 GT (512mb) to the 460 GTX (1 gb).
Not sure I can SLI on the x38 - I've seen conflicting postings in different places. But I'm be okay with just 1 460.
The 450W dedicated power supply was the smallest I'd found since I realized Dell doesn't provide a bigger power supply for this case and there's little chance one off the shelf will fit either. Then I ran across dedicated power supplies for the first time. But you've shown me if I can find one in the 320 - 350W range, that would work and would probably be cheaper.