r683 or whatever, right? remove any enablers, (NVInject, Natit, NVEnabler, etc) and reboot, and at the disk selection screen, (chameleon screen) type "GraphicsEnabler=Y" without the quotes, and if it works, go into your chameleon preference pane in system preferences, and check the box next to 'graphicsenabler' and click back then you're done. thats how my 7900 was picked up, which is obviously a tad newer than your 7800, so yours should work. good luck

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Hack Pro:
AMD Athlon 64 Dual-Core X2 5600+ 2.8GHz
Asus M2N-SLI Deluxe Motherboard
8GB ram (4x2GB DDR2-800)
XFX Geforce GT240 1GB DDR5
500GB SATA HD (Snow Leopard 10.6.6)
250GB SATA HD (Windows 7 64bit)