I run a 2 monitor setup. My main is a 20" Viewsonic that's about 18 months old and my secondary is my first LCD, a 4.5 year old 17" Hercules Prophetview (one of the best 17" LCD's in its day). When I got my Viewsonic and hooked them up next to each other, it is like night and day difference in quality. The 1000:1 contrast ratio and higher brightness specs are very visible in the Viewsonic. Also a 5ms response time is virtually no ghosting as compared to the 20ms of my first LCD.
With the money you saved on having the computer built for you, a 22-24" LCD will make it even more worth while.