If you a running a small website, server specs shouldn't really matter too much. It doesn't have to be ultra fast and as long as it has a decent amount of memory, it should run fine, providing of course, that the hardware inside the box is behaving.
So for a new server for what you require: Custom build one with parts from your local shop. You shouldn't need to break the bank, and should find that you will be able to build one for what you want, at a low price.
Another option would be to go with a hosting company who take care of everything for you. But that doesn't give you the fun and enjoyment of building and maintaining your own server
I'd try and be more helpful, but it's 6am here hehe
Tell me how you get on
Edit: Missed the whole point of this post haha. To be honest, i'm not entirely sure what the difference is. I've never been a fan of Dell's products. I'm guessing the powedge is a rack mountable server and the 'Desktop' is a desktop server, pretty much like whta you already have, but with a few added extras.