The new Vista computer I have is a Dell Dimension E521 (one of the ones they're selling in Wal-Mart now). I knew Dells can sometimes have screwed up PSUs that fit in such a way that you can't easily fit a regular ATX PSU in it. So I went looking around on Google for a bit of info.
This was one of the first results and it caught my eye, so I read further. The guy said that Dells need an adapter to connect a regular ATX PSU to the motherboard.
One of which he linked too.Now I've taken a close look at the motherboard in this Dell (kinda had to to install the 4 gigs of RAM) and I can't see a little connector anywhere near the main plug in for the PSU like in the picture shown. It looks like a regular old 24 pin ATX PSU plug.
Does anyone have enough experience with Dells to say that I could just plug any old ATX PSU in without having to fear breaking something? Does anyone even know if Dell still uses proprietary PSU connectors? I've thought about pluggin in the old PSU in the old computer in to see. But like I said, I don't want to blow something up.
Before I answer, let me just say that Dell PSUs are way under-rated. Your 521 probably came with the standard 305w. I personally know a guy (and you can read his post on the DCF, search for "8800GTS answers") who is running an 8800GTS on the stock power supply with no issues. So if your reason for upgrading is simply a power concern, you may want to reconsider.
Now to answer your question. First off, the yahoo answer is a bit misleading, since your case is a single slot and the 8800s require a dual slot. Granted, it can be done with a simple case modification, but it's not plug and play.
As to the actual connectors, your board will accept a standard ATX PSU. Dell stopped using proprietary supplies around 2000. I think ZiggyDoodle may be referring to the fact that sometimes it's hard to find a unit that fits the case, and that's where PC P&C comes into play. They use a standard size for the Silencer 470 which will fit the E series cases. They also are damn reliable.
If you don't want to take my word for it, and are intent on being 100% sure, read the manual for the PSU you are considering, and the manual for your Dell. Both will show the voltage for each of the 24 pins, and you simply need to make sure they match.