|"shouldnt it be weird if its the powersupply?"|
I took a stab at it because you supplied us with zero info about your system. I'm not convinced you have a good PSU though. From what I could find, it has 23A on the +12v & 40A on the +5v? The high +5v amperage is a trick used by PSU manufacturer's to falsely inflate the overall wattage rating of the PSU.
I gotta ask...why would you upgrade from an 8800GTX to a GTX260? Seems like a waste of money to me. I'll doubt you'll see much of an improvement.
As for your statement about updating the driver - this is NOT a driver issue. Drivers are used by Windows. This "problem" is happening before Windows loads. When the post screen becomes visible, is it right from the very start or does it become visible during the post process? Could it be that your card is just slow to power up?