Still looks to me like the 8600GT is your best bet for a minimal investment. And the price has gone down since you started this thread 
If I had a new Dell, still under warranty, there is no way I would replace the case or power supply.
Since you have the PCI-E power connector, you might go to a 8600GTS for about 20% higher performance than the 8600GT, if you can find one that is fairly-priced.
Or wait for the ATI HD3850 to come out, next week, (it will destroy the 8600GTS for not much more) and the prices of the 8600 series will be quickly forced down.
And I agree that a 8800GT on even a good 300W PSU is asking for trouble.
If I had a new Dell, still under warranty, there is no way I would replace the case or power supply.
Since you have the PCI-E power connector, you might go to a 8600GTS for about 20% higher performance than the 8600GT, if you can find one that is fairly-priced.
Or wait for the ATI HD3850 to come out, next week, (it will destroy the 8600GTS for not much more) and the prices of the 8600 series will be quickly forced down.
And I agree that a 8800GT on even a good 300W PSU is asking for trouble.