It's important to know exactly what you are planning to do in regard to upgrade.
Your Dell E310 has a Primary IDE controller that your Primary HDD is connected to. There is a slot right next to this HDD for a "secondary" IDE HDD.
It also has 2 SATA connectors ont he MB. Don't know if you have a CD or DVD drive, but the diagram of your computer shows they are located near the bottom of the case so they are setup to be connected as SATA devices.
If you want to replace your old IDE HDD with a larger one, that would require you to reinstall everything from your recovery discs, a BIG job.
If you want to just add a second "backUp" HDD for additional storage, that is easy as it would be placed in the additional slot next to your present HDD, and the ribbon cable and 4 pin molex power plug will be right there. You'd be ready to go.
Pretty much any IDE (Parallel ATA drive) HDD will work. The largest IDE drive I've seen is 500GB, and they are inexpensive and still available.
Here is a nice WD on Amazon: Western Digital Caviar Blue WD5000AAKB 500 GB internal 3.5" hard drive, and there is a 320GB for $20 less.
You have to be very careful you get a 3.5" IDE 7200 RPM model, in that they have very similar SATA and Notebook models that are not what you want.
You could add a "backUp" SATA drive on one of the SATA connectors, but almost certainly you would have to add a SATA cable and SATA power cord connection to run them up to the HDD slot near the top of the case. This would be a SATA drive, which is different.