See if there's a kext for your card. It might help. Because OSX isn't communicating with the card quite right, causing the Grid bug but otherwise clears tests.
Question, though...have you tried any games on it? Depending on the tests, they may not be fully proving things.

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Custom Hackintosh...AMD Athlon X2 5000+ (2.6 GHz) on GA-MA78GM-S2H...Linksys WMP300n Wireless...ALC889a sound...ATI HD 3650 PCIe...1 GB DDR2 RAM...320GB Western Digital SATA + 200GB Seagate IDE HDD...Rosewill DVD-RW IDE...Running Kalyway upgraded to 10.5.7...All Fully Functional