diadokmai, before you do any serious OC'ing, we haven't inquire one very important data that seems to be overlooked... the temperature.

Tell us what's is your IDLE ~ LOAD temperature of default setting (non-OC)
then for sure tell us the IDLE ~ LOAD temps when you OC the card.

You can run Furmark OR use Speedfan which have the graph that logs your GPU temps (or download GPU-Z, which also has an option to logged in the GPU data)
THEN try 10 mins of the most graphic demanding game you have and see what maximum temperature does the GPU hit (both in OC'ed and non-OC'ed that is...)
The difference running Furmark and gaming (for gathering data) is that Furmark is a synthetic 'stresser' and gaming is real-world testing, so Furmark you get you higher temps reading than normal gaming because Furmark loads your GPU 99.9%, and the average games will load your GPU around 80%-90%.

Anyways I use to owned an XFX 9800GT (600mhz/900 DDR3) exactly a year ago,
When OC'ed ( 700mhz / 1000 DDR3), I think the idle temps was 58c-60c and the load temps was max 75c using aftermarket (Zalman) Cooler.

Other comments:
>If you were happen to tweak the PCIE voltage in BIOS, I dont recommend going past '104' with the G92 (9800GT chipset), if you past 104-105, the risk comes in bro...
>Flashing the bios is easiest way, but also easy to screw up if you are uncertain of what you doing, usually from typo when inserting commands, etc.