If you want a Mac Mini/Dell Zino form factor, and if you're in the mood to DIY a machine, you could go for a mini-ITX machine. You'd get the form factor and performance you're looking for. Techreport had a guide for such a machine not too long ago:http://www.techreport.com/articles.x/17102/10
They priced it out to $510 with the integrated graphics. The Zotach 9300-ITX has a PCI-E 2.0 x16 slot, so just throw a Radeon HD5770 in there and you can play anything under the sun at 1280x1024. I've heard a Radeon HD5850 can fit into the Sugo SG05-B case as well, but you'd probably want to wait for Zotac's new 450W mITX PSU to come out before trying that. Keep in mind that guide is slightly outdated, so you could probably do a bit better now component-wise (for example, the latest revision of the 9300-ITX is the GF9300-I-E
which fixes several issues from previous revisions, like Wake-on-USB and lack of internal SATA ports). Also, because that uses the Geforce 9300 chipset (which is just an underclocked Geforce 9400 chipset), you SHOULD be able to Hackintosh the resulting machine.
DFI is also releasing a P55 m-ITX board soon (LANPARTY MI P55-T36
) if you want something for LGA1156.