Even a modest card in the <£50 range can display an image across a 1080p resolution, it becomes a different ball game though when you start gaming at higher resolutions as a more powerful card is then needed.
Whats your budget mate? There's quite a bit of difference between a GTS250 and 4870x2. A GTS250 can game at 1080p, albeit maybe not at high settings and would probably depend on the game also. The 4870x2 can easily game at 1080p with all the eye candy turned up, but make sure your PSU is up to it, its a dual GPU card and is power hungry. Its expensive too and quite hard to find these days, for the price mark you may be better off with something else for the money - 5850 perhaps?
It depends on your budget though