Not that I necessarily recommend the hackintosh route, but if you had 10.5 running, 10.6 should run as well. I had my system moved to 10.6 within a week of it's release.
I had an ATI 3870 which was supported in 10.5 but ceased to be in 10.6 - a shame, since it outperformed the nvidia cards in core graphics by a large margin.