6800XT= The 6800XT AGP has 8 pipelines like your 6600GT and has a core clock of 325mhz and a memory clock of 600mhz, and has a 256-bit interface, and performs around the same as your 6600GT. There's also this overclocked XFX version which is basically a 6800GT with 8 pipelines instead of 16(here:
http://www.newegg.com/Product/Product.asp?Item=N82E16814150158), and sometimes the pipelines can be unlocked.
Plain 6800= The Plain 6800 AGP has 12 pipelines and has a core clock of 325mhz and a memory clock of 700mhz if it's a 128mb model(The asus 512mb model has a core clock of 540mhz, from newegg's specs), and under lower settings, it performs the same as the 6600GT, but when you apply AA and AF, the 6800 will begin to pull ahead of the 6600GT.
6800GS AGP= The 6800GS AGP is basically an overclocked Plain 6800 which performs between the 6800 and 6800GT, but doesn't perform like a 6800GT unlike the PCI-Express 6800GS.
6800GT AGP= The 6800GT has a core clock speed of 350mhz and a memory clock speed of 1000mhz, has 16 pipelines, and is the second fastest card in the 6800 series, and was a really good card back in 2004, and still is a pretty decent card, but it won't play the latest games like oblivion unless you put them on 1024*768 and medium with no AA and AF.
6800 Ultra AGP= The 6800 Ultra is basically an overclocked 6800GT and doesn't offer much performance over a 6800GT, so it won't be running the latest games any higher than a 6800GT.
6800LE is basically an earlier card before the 6800XT, and it's practically the same thing.