If you throw in a decent PCIE videocard you can do some gaming on it...
I would say feel free to throw in a low-medium range card if you have one in stock... like a radeon 6670 or 4670... Nvidia GTS250...
Investing on a high end videocard for this machine is ill advised.
2x E5405, as much as it has an insane amount of cash memory... is nowhere near an old I5 750 or even a Phenom 955
In Intel® Xeon™ E5405 share the same basic micro-architecture as the 1st generation Intel Core™ processors. So with this quad core processor in a dual processor set up you should be able to run a game without much problem. There are only 2 issues that I can see. They are the PCI-e 16x slot to put the video card in and other is the low processing power for most modern games.