...the guy at John Lewis suggested it...
That's almost as bad as taking advice from people in PC World.
But yeah, any graphics card made in the last few years will be able to support nice, high resolutions. To be honest, unless you've got a massive screen, you're unlikely to even use resolutions that some graphics cards can do. It's more important that you check what connections your graphics card has, and buy a monitor that uses them (or at least, buy an adaptor). DVI (usually a white connector) is the most common at the moment, with VGA (a blue connector) being the older, analogue connection. Fancy schamncy cards also have HDMI outputs as well, but you won't need to worry about that unless you're playing blurays on your PC.
