I totally agree. The X800GTO is about halfway between the 6800GT and 6600GT IMO. The closest performing GF6 to the X800GTO is the AGP 6800GS, (although in PCI-e it beats the GTO as it's clocked higher than AGP version).
Also, I too would hesitate to recommedn either. The 6600GT is usually 128MB, which is not supported in some games now. The X800GTO is SM2.0, which is also not supported in some new games.
Not sure on prices in Australia or if you are even looking new or used, but I'd look for one of these - HD2600XT, 8600GT, 2600 pro, X1650 with GDDR3 (XT>GT>pro), 7600GT. Find your prices and ask again including the games you want to play and the system specs.