Hi there,
What graphics card and monitor you should go for really depends on what you are doing and what your budget is.
First thing, in terms of gaming performance, it isn't the physical monitor size that determines how well the GPU will perform - it's the resolution. At the moment you can pick up a HD 5850 card for £117 that can handle pretty much any modern game very well - so long as you are running at a resolution of 1080p or less. You can spend more on a gaming graphics cards, and this will allow you to increase the settings a bit, achieve even higher framerates and last longer - but imho the value of that 5850 is tough to beat.
As for the monitor not only is the resolution important - but so is the panel technology, true response time, input lag, black level, viewing angles, panel uniformity, colour accuracy, static contrast ratio, colour gamut, backlight type, connectivity options, stand quality, build quality and warranty.
