overclocking probs

[marafice]

i've got a strange problem.
iive been running my e6400 at 2.66 for weeks now with no problems until i upgraded my v/c from a 256 meg 7800gt to a 512meg 7900gt(factory o/c).
now i cant overclock at all. the system hangs at boot and i have to reset bios.
i'm thinking powr supply but not sure. system speds are:
e6400
giga s3 mobo
2gigs kingston 533 ram
2x sata 2 hd
dvd burner
palit 7900gt@ 550-1320
onboard sound
antec trupower 480watt psu

any thoughts?

 

[marafice]

yeah, tried that redid everything except easing my ram timings-kingston value ram but i'm not sure what the stock timings are. funny thing is if i set everything to manual but then leave all settings at stock, the comp still won't boot. thats why i figured psu cause it was overclocking easy until i changed the v/card. all other settings were working fine with the 7800gt

 

[darkstar782]

The 7900 is a 90nm part and uses much less power than a 110nm 7800.

Is there a PCI-E frequency lock that you have missed? It should be at 100MHz.

I assume the system works fine @ stock?

 

[marafice]

works perfect at stock and i do lock the pci-e at 100. i can't understand it, with all settings exactly the same the only difference is the v/card. if i put the 7800 back in, bingo o/c works fine. put the 7900 back in, nothing. a mystery
edit: i also updated the mobo to the newest bios--no difference

 

[marafice]

update-- reflashed the bios again and now i can o/clock, though not as stable as before so matbe some mobo incompatibilites with 7900 cards. will see how it goes as the bios's mature. thanks for your help in the meantime.
steve