OK as mentioned, is this Just for CS4?
If so then you don't need anything high powered, nor waste your money on a Quadro.
If you're gaming, then that DELL power supply is more robust than a generic 300W supply, and it should easily handle an HD4770 which would give you better performance than that GF9800GT at lower power consumption and with more features supported.
But if all you're doing is photochop and no gaming, then a GF9500 or HD4550 would be fine and still give you a bit of OGL and OCL power for basic acceleration, and cost you less, and even an HD4650 would be worth the extra few bucks.
Also if you're not gaming, you might want to also consider a passively cooled card that doesn't have a noisy heatsink fan assembly.
However longer term, or for gaming, something like an HD4770 would be a good choice.
And you don't need a new PSU if this is all you're doing.