first off, stay away from that 6800, its not even a 6800, thats more of a 6600gt, it has 128bit, which is crap... 512 mb memory will be worthless with that small interface... the x800gto will be fine, or a 6800GS if u can afford it, those are closer to 230 tho on agp... cards that dont support sm3.0 wont look bad, but they wont look as good as with sm3.0, provided the card can handle the load on higher settings, a 6800gs wont be able to. in the interest of saving money ill recommend an x800gto, if u feel spending another 30-40 bux isnt a big deal then the 6800gs will be a bit faster and i hear those overclock like no other (then again the 800gto overclocks pretty good too)
-x800gto or
-6800gs
things to look for when considering a card
256 bit interface
256 gddr3 memory (ddr2 uses more power and wont let you overclock as high
sm3.0 (in this case u may not be interested in it, but all 6XXX series nvidias support it