What GPU can I use on my E520 without having to change the 305W PSU.

If you do a search of this forum you'll find about 100 answers to your question, if not more.  This is brought up almost every other week. 

 

To answer your question briefly though, I think the stock PSU in the E520 would be able to handle the mentioned video card.  However, it depends on how many other upgrades your system has among other issues.  I have the 512MB version of the Sapphire X1950 Pro in my E520, but I upgraded my PSU by choice. 

 

Check out this site:

http://www.extreme.outervision.com/PSUEngine

It will help you calculate the maxium watts put out by your system in a particular configuration.  Additionally, IIRC the Sapphire X1950 Pro cards recommend a PSU that has 30 amps continuous power (Amps are another aspect of power consumption).  I do not think the Dell stock PSU is rated for that, however I believe there are users who are running that card with the stock PSU, and not having major problems.