I just installed a GeForce 6800 GT, it will work for a moment, then the screen fills with lines and it freezes and even loses signal to the monitor. Is this a driver problem?

it is not a driver problem.

it is due to over heat.

buy another fan or open the cpu cabinet.

i suspect youyr graphics card is heating up and shutting off to prevent damage. windows will also shut it off but usually repaorts that it did. here is my reccomendation:
1. open side case and place a fan right nest to it. if this makes it work then heat is definitely trhe culprit. if so install a video card cooler or another exhaust fan.
2 turn all the graphics options down on the game. shut anti aliasing off and all options to minimun. then try to play if it still shuts off then your card may be on its last leg from over heating. the 6800 gt was ok when it came out but todays games require more. in fact the newer games won't run on a 6800 gt.
the big thing is heat. the standard 6800 gt just doesn't get rid of enough heat. in fact the 9800 gt i had overheated and burned out. i stay away from gt and now use gtx cards. they have a better cooler and ususally the vent outside of the case.

try removing 6200 completly,and then put on 6800 and instal that gpu first.....

No, you dont need any power to the Graphics card.

But make sure that the monitor is connected to the new graphics card that you have installed and not on the on-board VGA card. That should do it for you !

Usually the problem with windows 7. if you have 2cables connecting from youre monitor to the graphics card, unplug one of them, it worked for another guy

Try to find out what the AGP voltage is on the motherboard.
Some graphics cards use 1.5 volts and some use 3.
If it's a 1.5 volt AGP slot and you plug a 3 volt graphics card in, it either wont work or you could permanantly damage both card and motherboard.

what u meen on the screen dint understand ya

Probably not. There is not quite enough information in your post to really help, but from what I'm finding on the GeForce 6800 GT, it is only available in PCI-Express and AGP 8x (AGP3.0) Versions. Your motherboard only supports AGP 1x, 2x and 4x (AGP2.0) cards.

Could you post your system information as well? CPU, memory and so on.