if you want ATi get an x850pe or x850xt. the x1800s and x1900s and x1950s are also good but often cost more, they use DirectX9.0c unlike the x800s and x850s. that isnt a big deal yet, but it will be.
if you want Geforce go for the 6800GT, 6800ultra or the ultra extreme if you can find one. all the 7800-7900-7950-8800 models are good but apart from the 7800s you'll struggle to get one for £100. all these use DirectX9.0c. nothing inbetween the 6800 and 7800 is worth looking at.
dont assume you can rate a card by its model name or number, for example an nvidia 6800 is far better than a 7600. an ati x800 is far better than a x1300.
personally i think these days you want at least a 256bit memory interface and 16 pixels per clock. anything less and you'll end up replacing it in the near future.
i got a new GeForce 7800 GS for £35 on ebay so you can get a good card for little money, it just requires patience and a very long watched items list.