hey there i have the nvidia 8400gs 512mb card and i have no problems i used ideneb 10.5.8 lite edition and used the NVenabler for the 512mb card everything works great.maybe try that.

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ideneb OSx86 10.5.8 lite
acer aspire m1100
amd athlon 64bit dual core 4400+
4gig of ddr2 ram at 800mhz
320gig hd
1ide dvd rom/cdrw and 1 sata dvd/rw drive
22in insignia lcd tv/monitor.
nvidia 8400gs 512mb