Easy one - NEC MultiSync® LCD20WGX2 20.1" Widescreen LCD Monitor - £422. OcUK are temporarily out of stock though.
It's the quickest widescreen around, great black depth and viewing angles, not supposed to be the best screen for movies, but for games it's the best widescreen.
The Philips 200W6CS is worth looking at too, also the Viewsonic VX2025 now it's come down in price.