You need to make sure you have a motherboard revision that can support the additional voltage to the gpu interface. There is a sticker in the memory bay that will tell you what revision you have. If you ordered your laptop long ago when the 6800go was the only gpu offered, it's likely you have an early revision that won't support later cards.
In addition, you need a 220W AC adapter instead of the 180W that came with earlier revisions.
rjtech.com should be able to help you figure this out. If they can still get the later motherboards (the d900t has been discontinued) they can sell you one for what I've seen here for $480.
If not, you can get a d900k motherboard, that's for the AMD processor, so you'll also need a new processor.
It might be worth it for you just to sell your 7700 and buy a new system with all it will cost to upgrade it... The newer core2duo systems are much faster than even the desktop pentium platforms.