The 4890 will fit fine.. The real problem is installing some of the big tower CPU coolers.. Also the corsair TX 650 will easily power a single 4890 (may be even two, not sure though).. For a skeleton though, you might want to look for a modular PSU.. Your choice..
crap...sorry guys...its not a H4890, I already have on of those, it the HD 5970 from the link.
he hd 4890 is 23 cm in lengh, and the HD 5970 is 30 cm in length...im not sure if it will fit as the motherboard has sata connectors positioned in line with the pci-x slots. also cant measure as i dont have a ruler or anything