You'll need a fast video card to drive a large monitor. You'll need an even faster video card to max out a game's eye candy on a large monitor.
All LCD monitors have a native resolution--the one resolution that doesn't require pixel interpolation--that should be used to obtain the best picture. The larger the monitor, the higher its native resolution. Large monitors with high native resolutions require vast assloads of graphics processing horsepower in order to achieve acceptable frame rates.
While you can force an LCD monitor to display non-native resolutions at the expense of image quality, it's obviously nicer having a video card that's up to the task of driving an LCD at its native resolution.
Here are some general guidelines:
Up to 19" (1280x1024, 1440x900) - Radeon HD3850 (if you're on a tight budget, you could probably get by with a GeForce 8600GT(S) for a little while).
Up to 22" (1680x1050, 1600x1200) - GeForce 9600GT or Radeon HD3870 (if you're on a tight budget, you could get by with a Radeon HD3850).
Up to 24" (1920x1200, 1920x1440) - GeForce 8800GTS-512MB (if your budget limits you, you can grab the slightly slower 8800GT-512MB).
30" monitors (2560x1600) - 2X (SLI) GeForce 8800GT or GTS (512MB), or the new Radeon HD3870 X2 (two HD3870 cores on one card). If you can afford a 30" monitor, then buying two performance video cards shouldn't be a problem.
The creme de la creme of Socket 939:
Opty 185 @ 3.2GHz
SLI'ed GTS-640s, both flashed to 625/1458/1950
4GB PC3200
Blu-Ray/HD-DVD, X-Fi
A8N32-SLI Deluxe
3DMark06: 13896
