My question is that are these combinations with the Dell XPS 8300 compatible?
My budget is $250
Will the 500 Watt make my Dell XPS 8300 fry from Over Wattage?
Should I just stick with my 460 PSU, or upgrade to the 500?
And last but not least, Will the 460 PSU be compatible with the video card that i'm planning to buy?
I would stick with the Dell PSU it is underrated at 460watts and is better than many generic 500watt PSU's. It has sufficient amperage on the +12volts to run the card.
It does not matter how big a PSU you have the components will only draw what the need from it, will not fry anything.
I suggest go with a GTX 660 or HD 7850. They perform better than a GTX 560 and the two cards consumes less power (only one 6-pin power connector) than a 560. So, the Dell PSU will easily be able to handle it.