Don't forget if you want 3D you will need to factor in buying Nvidia Vision kit (1 or 2) as well as needing a Nvidia graphics card to use with the vision kit as these dont come with the BenQ XL2420T.
The alternatives if you have an ATI card or dont want to buy a vision kit would be the Samsungs S23A750D, S27A950D or there is also a cheaper version of the S23A750D but can't remember the model number as they come with glasses etc but over Jamy's budget.
As for Jamy with strict budget you do have the option of LG D2342P 23" (
http://forums.overclockers.co.uk/showthread.php?t=18377655) while not a 120hz 3D monitor and you may need to track down the correct glasses if it doesn't come with the passive monitor 3D instead of Cinema 3D glasses, but for the price the 3D does work with the tridef software it comes with and will run with any graphics card and in 2D its still a reasonable monitor