You do not want the Sapphire GTO, but the GTO2.
That is the unlockable version, and it is £140 odd.
i think it is a no brainer to get the sapphire gto2 if you can. The overclocking potential is enormous, and while you are not guarenteed to get x850 speeds, you are just about guarenteed to reach at least x800xl speeds, which is a £200 card.
however like all the x8xx series, it does not support shader model 3, and so the image quality will not be as good as an nvidia 6 series card.
but the 6 series cards are beginning to struggle with new games that are very intensive anyway, and with the gto2 at least, you will get good framerates for a good while still.
I don't think you can get a better value card than the x800gto, but it is up to you whether you want to spend the extra £35 on the unlockable gto2.