E8200 only runs with 1066 fsb

You probably need to update your BIOS.

The E8200 probably wasn't available when your machine was made.

Go to Dell website, and download the latest BIOS.
Or if this link works, use this:

http://supportapj.dell.com/support [...] eid=299050

Edit: this also improves fan profiles, so you may find your computer is quieter after doing the upgrade as well.

Oh ok excellent.

To me it sounds like it is running 1333 FSB (well 1280 but its close, 320Mhz * 4 = 1280) but either it does not have (or needs to be manually setup) the memory multiplier required to run the RAM at 800 (so it drops them to 640 (320x2)).

Someone more familiar with a Dell might be able to give you information, I haven't used a Dell BIOS and I'm not sure what features you get. While you're waiting for better responses, contact Dell support.

If the memory frequency is 319Mhz, then chances are that's your FSB frequency as well.
FSB1333 = 333Mhz x 4

It's your memory that is running slow. (319Mhz instead of 400Mhz)

(Edit and the mix of RAM frequency, RAM speed, memory multipliers, FSB speed, FSB rating, etc etc can be all very confusing, I'm just about to leave for the weekend, but if anything confuses you just ask, someone else should be able to give you more detail).