Re: Best desktop to get for under $500?
Really depends on what you are going to need your tower for and the support you need. Sure you can build your own but you don't usually get the best after sales but then Dell aren't that great anyway.
If your just going to be using it to surf the net, play a few games and the odd bit of home computing (ie the odd letter) then what you have specified is fine.
If your going to do more and if its for business then the first thing I would do is upgrade the operating system. FWIW XP Home is not going to cut it for business use. The next upgrade you will probably need to look for is getting a DVD writer as that is the standard these days. Then and only then, if your into playing really hard core games or are going to run cutting edge applications upgrade the processor.
I don't know how much they are in the US but in OZ you can get a pretty decent HP Pavillion for that price especially as you don't need a monitor or speakers.