Question: I want to try Ubuntu on a desktop I just got, `DEll Optiplex 755`. It is a bit old, I think 2007. Will the new version of Ubuntu work on it? I am most worried about RAM and hard-drive space, and whether the graphics will work.

Answer: 
Refurbished Optiplex GX755

I did it for Ubuntu 13.04 and it worked really nice 
RAM upgraded to 2Gb,then 4Gb as a test
Video upgraded by using an AGP video card 256Mb,3D (Unity,Cube3D works fine)
HD 1 Tb

Now I'm about to upgrade Ubuntu to Saucy Salamander (13.10), without expecting a "downgrade" of the performances.

Ubuntu yours
If you have enough memory (ca. 20 giga free): make a different partition and install ubuntu on that partition. You can do this with Gparted (<http://gparted.sourceforge.net/download.php>). Then you have a dual system. I advise you to install ubuntu 12.04 (the long time edition).