Basically nobody knows what any of the x1000 family cards can do at the moment, as they are on very early drivers and are very unoptimized. I am currently very interested in how this turns out , as the x1800xl has the potential to out perform both the gt and maybe the gtx in my opinion, but it will take time.
When you look at the x1800 xt and the xl, both the xl and the xt have 16 pipelines, and i believe use the same memory chips, however the £100 difference in price is due to the xt having it's core clocked 150mhz faster and memory 400mhz faster. So really if you are into overclocking then i would guess that the xl could reach xt speeds?
I can't remember where i read it, but some company in sweden overclocked a x1800xt to over 1000mhz core clock frequency! This was of course with an unlimited budget and liquid nitrogen cooling system, but there is an awful lot of potential in the x1xxx family that has yet to rear its head, i feel.
Mid November i believe that nVidia is realeasing a 7800 gtx with 512mb memory to compete with the x1800xt as soon as it hits the market, so if i were you i'd wait for that to come along so the prices of the other cards fall
Of course the 7800 gt represents the best value of any card at the minute i think, for around £250 (some x850's are more expensive) you can get massive amnounts of fps and effects for great value.
Historically ATi tends to beat nVidia when it comes to effects, and now that both makers are fully shader model 3'd (new verb
) it is a level playing field.
My recommendation: The x1800xl has alot more potential than the 7800gtx does at this moment in time.
n.b - HOWEVER if you are planning to use a ATi Crossfire or nVidia SLi motherboard, then obviously get the respective manufacturers card
