The 7800 is one of the newest cards from Nvidia.
It is alot better, and is just as good as 2 6800's running together, and that explains it very well.
RAM though doesn't matter that much in games, and 256mb is quite alot considering most cards only have 128mb. 512mb is becoming standard now though, and we'll probably see the 7800's coming out with 512mb soon.
Most games wouldn't take use of it though. You wouldn't get more speed, etc.
But, it'd allow you to use less compressed textures, which may make textures look slightly better within games.
WHAT IS BETTER:
Even though they run at almost the same clock speed (50mhz difference), it gets its advantage from the extra pipelines and extra Vertex units to help render graphics faster.
20 Pipelines to be exact on the GT version, compared with 16 on the 6800GT. The GTX version of the same card has 24 which would make this version slightly faster again.
The Vertex units also help, with 7 on the 7800GT compared with 6 on the 6800GT.
Look here for comparisons with games:
WHAT YOU MAY FIND:
You may find though that the 6800GT and 7800GT might compare the same within games sometimes. This is only because of a CPU limitation, where the CPU just can't handle what the GPU is trying to do.
So expect when you buy one of these cards, that as you go up in processor spec, games will also run faster.
The 7800GTX is the ultimate in 3D gaming.