I don't have a crossfire capable motherboard, mfarrukh.
Jennyh: I've been trying to get a 4890 but I can't find them in stock anywhere, and overclockers.co.uk want to charge me extortionate shipping amounts. I got hold of an XFX 4890 earlier this week but it was honestly rubbish, 20% performance drop from what it should have been. Also I don't mind about the lack of dx11 or eyefinity, those really aren't priorities for me justnow.
Jennyh: Honestly, I'd rather not buy a rebranded Powercolour (as that's who VTX are), and not from scan either, I have a friend who's currently fighting them in the courts over their terrible business practices.
Several places in the UK have the HD4870 for close to, or under a hundred quid and it is the direct competitor to the GTX260.
For more power the HD4890 is around for under your budget but it is rather too expensive for its performance improvement over the far cheaper HD4870 unless you are prepared to overclock it.
A few places to look: Misco, Overclockers, Scan, CCL, Ebuyer, Dabs, Pixmania, Microdirect, and Novatech-but there are others!
Right now the fastest card within your budget is the HD4890.
Of all those discussed or suggested, the HD5770 is, actually, the slowest and suffers a performance hit at higher resolutions due to its 128 bit memory interface.
Trouble is that ATI currently have a large hole in their range between the 5770 and 5850 and the GTX275 (next step up from the GTX260) is both in short supply and fairly expensive.