Your best bet truly is to image the drive that came with it (unless you also got whatever installation media Apple shipped with it) and then put a new drive in to install some other version of Mac OS X with hackintosh techniques. The version you install really doesn't matter if you're just going to hackintosh it, and at that point, you may even have more luck with a better video card, unless it is, as noted, a board with a PCIe slot that only supports the ADD2 card.

If you can get 10.8 or newer on it, that's what I'd personally recommend running, but 10.4.11, 10.5.8 or 10.6.8 (or possibly 10.7.whateverthelatestversionwas) may end up working best with the integrated graphics. 

As always, I personally recommend against hooking a machine running a non-current version of Mac OS X to the Internet, for reasons. It would be a great darknet machine or if you just feel like having an install party, a platform to play with as many different versions of Mac OS X as possible.

General DTK question: has anybody looked at operating systems other than Mac OS X on them? Windows 7, 8.1U1 or 10TP would probably do really well on this thing, especially if the Pentium 4 CPU in it has HyperThreading and you can get 2 or more gigs of RAM in it. (915 chipset should happily run 3 or 4 gigs.)