I have a Dell Dimension E521. I'm buying parts for a whole new computer that I plan to build soon, but I want to put some into my current computer for the time being. What power supply would fit my dell and handle a HD 4870, but would also be transferrable to a new case that i plan on building soon?

I'm not sure about your DELL E521 PSU or what units could replace that PSU, but this unit installed in your DELL would power your 4870 if you are concerned about standard PSUs not being compatible with your system. I'm not familiar with DELL systems, so it's just a suggestion.

http://www.newegg.com/Product/Prod [...] 6817153069

You can use almost any standard ATX psu, you just have to make sure to get one that isn't to long for the case.
The Corsair HX 620 and 520 are a couple of favorites, due to thier modular design.
By the way, the 4870 won't fit in the E521.

Im not sure if u can use a standard ATX.

I opened my parents computer (Dell XPS 710) and it was using a non standard PSU. It had no cpu connectors.

Maybe the new ones are different....

The PC Power and Cooling designs some dell power supplies.

Your parents 710's power supply is proprietary, it uses both a 20 pin and a 24 pin mainboard connector instead of a 4 or 8 pin cpu connector and the standard 24 pin ATX connector, the E5xx series use a standard ATX psu .

^Yup check this out:
http://www.pcpower.com/products/power_supplies/dell/

Years ago that mattered, since todays Dell's use a standard ATX psu and have an open back design the lack of a switch on the PCP&C Dell units doesn't mean much.