After looking up your specs and then checking the HCL over at osx86, it doesn't look very promising. I only found one board with a successful install using the intel 915 chipset. It was done with the same distro you're using though. Unless you upgraded the graphics at some point, the Radeon X300 is probably a lost cause.

Go to this HCL and look for the 775 Dual-915GL. It's the closest match to yours... Best of luck.

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HP Pavilion a6544f
iAtkos v7 (10.5.8), Asus IPIBL-LB 5.23 BIOS, Intel Quad Core Q6600, 8GB RAM (800mhz), 150GB Raptor-640GB-1.5TB, nVidia GTX260 Dual-DVI (Working), ALC888S (Working except SPDIF), Onboard LAN (Working)