Re: 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.
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.