X1600 is significantly slower card, it just happens that oblivion is badly designed and runs better on newer ATI architecture (in budget cards). Most newer cards in this price range are significantly slower too, so a benchmark might show similar, even higher performance at newer games, these benches fail to show what the important factor is- minimal framerates, and how these cards would compare if enough eyecandy were disabled to make minimum framerate acceptible.
In other words, forget benchmarks that show higher resolution and max eyecandy on newer games, that's not what you'll want to use on a $100 card looking forward in time.
6800XT is not all NV40 now, some have not been able to unlock the pipes. Even so, it's been reported several times that they o'c to around 430 Core / 1.2GHz memory, or higher, with most of that being practically a given and that or higher frequency, likely.
It is a fairly power hungry and larger card (length), if you have what you deem a marginally sufficient PSU it could be an added cost.
If only there were better odds of unlocking these to 12-16 pipes, it'd be a great overclocker's budget upgrade. As it is, on average it outperforms other $90 cards but as always, game demands keep growing, it's not a long term solution.