whats the difference

7800 is considerably faster.

6800GS= 555 million Vertices/Sec
33.6GB/second memory bandwidth

7800GS=600 million Vertices/Sec.
40.0GB/second memory bandwidth
 
which are u planning on geting. my mate has a 6800gt oc'd fairly high tho and he plays games a 1280x1024 on high details at good framerates