Try updating to 10.5.7 and then see how your games perform. The 10.5.7 update includes the latest OpenGL extensions which provides better support for that card. From what I understand the HD 4870 that shipped with recent Macs have been using a special build of 10.5.6 that enabled the card to work. However, performance has been noted as particularly buggy in some cases.

Good luck

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MacBook Pro 15" | Mac OS X 10.6 • 2 GHz Intel Core Duo • 2 GB DDR2-667 • ATI Radeon Mobility X1600 128 MB • 100 GB Serial-ATA
Hackintosh | Mac OS X 10.5.6 • 3 GHz AMD Athlon 64 X2 6000+ • MSI K9N AM2 • 2 GB DDR2-667 • NVIDIA GeForce 8800 GT 512 MB • ALC888 Audio • 300 GB Serial-ATA
iPhone 3G • 8 GB