Obviously a 20-24" widescreen is going to offer more horizontal viewing area..I believe a 22" is the sweetspot for being the same vertical height as a 4:3 20" yet widescreen. I know nothing about CAD however so a crt may actually be better for your purposes. I know that professionals that need accurate color either stick with a crt or get a good IPS lcd panel.
Widescreen offers more viewing area than 4:3 of the same resolution class. E.g. WSXGA+ (1680x1050) has more area than SXGA+ (1400x1050). Do not fall into the trap of comparing widescreen to 4:3 across different resolution classes. WSXGA+ has considerably less area than UXGA (1600x1200).
For CAD work, you should be considering UXGA (1600x1200) and WUXGA (1920x1200) monitors only. WSXGA+ is not worth considering as it costs about the same as UXGA but has far less area.
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