I'm not sure what they cost from visiontek (they tend to be pricey), but overall, the 4830 is just a little bit more. Its also a bit faster though (generally speaking graphics/gaming wise), but has better technology as far as video processing (at least so far as I have come to understand, I only briefly researched it). keep in mind if the 48?? series is too expensive for you, the 46?? series are going to take a hit performance wise for games, but should have similar video performance.
What do you mainly use it for?
About the power supply, assuming your dell takes a standard atx power supply (read the label and or is the power supply the same size as a standard atx), there is no danger or detrimental effect to going larger, no matter how large. The only disadvantage to going extremely too large, is due to efficiency, it will waste a little more power (as far as your electric bill goes).
Fyi, the power supply is very important (the entire system depends on it and a bad one can kill parts). A good 350 watt power supply will out do a bad 500 watt one. You can get good ones for reasonably cheap prices too. If you are not familiar with power supply's, defiantly ask for help (don't just go buy any 450 watt power supply, even if it is an expensive one).