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: 
Your friend here is a [Live CD](https://help.ubuntu.com/community/LiveCD) or even better a [Live Flash Drive](http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows)

This way you can try it on for size and see how it goes.
I'd go with Alex and suggest you try lighter versions of Ubuntu. Alex said Lubuntu, which I don't know, but I use Xubuntu daily on a 2006 entry-level machine and I like it. I also run it on a *1999* secondary machine and it's flawless. Some may say the look is spartan, I'd say it is modest and unobtrusive. 

Anyway the basic answer to your question is : "Yes, it will work well".
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).