I suspect that everyone else is waiting to see if you will use the forum's SEARCH feature (it sometimes actually works). This same subject comes up a lot at hardware forums. Dell, like most of the other brand name companies, prefers not to lose a customer once it has one, so they make sure you can't upgrade easily. For the most part, the power supplies and MB's are non-ATX standard when it comes to the connectors.
This makes upgrading more than oriinarily difficult. Aftermarket power supplies for Dell PC's are available, but the only place you can get MB's is from Dell. And the prices charged are anything but competitive. If you choose to change out BOTH the PSU and the MB, then you will find that not even the case itself is ATX standard. The front panel connectors to the MB are also proprietary.
At that point, all you have left is moving the drives. Chances are the MB is so old that the RAM isn't up to date, either. Practically a complete build. It would be cheaper to buy a new PC and just copy over the data files from the old drive.
