Re: 16 pipe 6800 ok for current-gen @ 1366x768?
I'm going to have to say this is cutting it close, performance-wise. However, since you are using AGP, you really don't have a better choice @ $100 than a 6800, other than not buying anything.
It depends on the game, but you are probably not going to be able to use AA, and might have to reduce details from high to low/medium.
I've used a DDR3 7300GT clocked at 600/1500. This performs about as well as a 16-pipe, DDR3, 6800. A 16-pipe, DDR1 6800 will perform about as well as a 7600GS.
If you want to play GOW PC, then yeah, I would take a step up. For Rainbow Six: Vegas (UE 3.0 game), the maximum playable resolution on DDR3 7300GT was 800x600.
Regarding 6-series vs 7-series, you should note that the latter is considerably more efficient and performs better in shader-intensive applications. Newer games are going to reveal the difference.