for agp.. i'd stick to 7800, 7600, and 6800, if you're on a tight budget, i suggest you save for one of those 3... they're top of the line agp and you definitely would not regret the wait for one of those 3 cards
if you can get your hand on a 512mb 6800 go for it. It seems the 512mb 7800gs from gainward does some crazy good, so maybe the 6800 may very well shock you as well
Dont get any 512mb 6800 because the card cant handle that much ram. It will wind up slowing you down in the long run. If you want 512 get something in the 7*** range of cards. I suggest either a 6800GS/GT or a 7800/7600 myself. Ultras take up to much power and becaue of the added v-core they die easily. My 6800GT is gonna hit the deck soon because i flashed it and the v-core is killing it.