Apple has reportedly seeded a new software update called "Snow Leopard Graphics Update" to developers. While the exact focus of the update in testing is unclear, it appears from the areas in which developers are being asked to focus their testing that it addresses several aspects of graphics performance including VRAM utilization and hot-plugging and wake-from-sleep issues. The general release notes claim only that the update addresses reliability of "graphics applications and games".
Check this thread for some extra info.


If you wanna test it out, It seems the update for the Mid 2010 iMac has some of these improvements already, here's the download page, you can try installing only the video related kexts and frameworks with pacifist cause it won't install if your system is not an imac mid 2010.
My 8400gs is performing a lot better with this (did some tests before and after but didn't save the data :P)

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CPU: AMD Athlon 64 3000+ @2.3Ghz - Motherboard: ASUS A8N-SLI nForce4 SLI - nForceLAN by eno - SuperNForceATA by Medevil - RAM:2 GB DDR 333Mhz Audio: ALC850 Video: XFX nVidia GeForce 8400GS 256Mb (0x06e4) QE & CI from Chameleon 2 RC4Ethernet: Realtek RTL 8139 Series (working out of the box) OS: Snow Leopard 10.6.4 (aryajuanda's guide) + Windows XP 64bits