I'd say your best size would probably be a 22", the 24" haven't quite come down in price yet(But if you are prepared to wait another 6 months, then I think the 24"s will come down), and anything bigger does really improve in terms of resolution past 1920x1200.
Arranged in order of Price:
Budget: ViewSonic VW2226BW
Value: ASUS VW222U
Features: ViewSonic VW2255
Coming Soon: Samsung SyncMaster T220P
Performance: ZALMAN ZM-M220W
24": Dell UltraSharp 2408WFP
Notable Mention: Samsung SyncMaster 226BW - This can be a great monitor however Samsung have been using different specification LCD Panels (this is a common trick used by other companies too), so some 226BW are different to each other, in short you want to try and buy the 'S' (Samsung) panel version only.
Check out:
http://www.behardware.com/articles/673-1/samsung-syncmaster-226bw-c.html