Well the main bit your looking for when to choose a company is what warranty they offer really. The other part is how far they have overclocked the card, and well if you're an overclocker, you can go above their overclock.
But if you not an overclocker, then look out for the two main figures (i.e. the core clock speed, and the memory of the card). Currently, XFX (like aVGA and a few other companies) have got 490 for the core clock speed and 1300 out of the memory. When you compare this to it's stock levels (400 core clock speed and 1000/1050 memory), then you see how much more performance you're getting.
I've had a look on eBuyer and Overclockers UK to find the Asus 7800GTX and can't find it, and so I was guessing you were choosing between htis XFX version:
and the Asus card (which I can't seem to find). So just look at the core clock speed and memory and see how much more you would be getting from the XFX version. If it's not cost effective, then go Asus
But take a look at this one:
It's a Gainward version (not the best I know), but for the specs it's got (440/1300) you'd pay over £50 more for the same from XFX or Leadtech
Just thought I'd let you know!