Adding to my previous comment:
Paladin is absolutely right:
Just in case there was any misconception, I am saying that I don't know what motherboard it is, however many of the LGA775 type motherboards I know of have the correct PCI-e slot (I wasn't suggesting the socket is what dictates compatibility) therefore it will likely be compatible.
I was also noting that I don't know your PSU. And therefore as mentioned you will need to check what PSU you require. Same goes with case size.
So just to clarify, I'm saying it IS compatible logically, just I wouldn't be able to tell you if your PSU and Case are acceptable. As Paladin says. the card recommends a 450W PSU, and I wasn't sure what PSU the Optiplex came with. Just to clarify, I was NOT suggesting that everything will be fine