Try to get at least a 17" screen. The 68 kHz models are quite cheap, for $400 the monitor can even be fine. Sometimes it can be useful to have two line inputs (BNC and D-SUB), e.g. if you have two computers or two video cards (Voodoo board and 2D card ). Personally I would get one with at least 86 kHz, this makes it possible to run 1280x1024 at least at 75 Hz if needed.

A 19" monitor is today the best compromise between a large screen and acceptable price. One of the best is the Idek Iiyama S901GT, it has over 100 kHz and a brilliant picture. Tom is using the 21" model from Iiyama. Of course high end users will rather go for e.g. an Eizo monitor, but the price for the Idek 21" monitor is quite good and the quality makes most 17" monitors look quite cheap.

TFT screens can be regarded as state of the art. The 14" types have become attractive but are quite small. All bigger sizes are still very expensive and cannot be recommended for home users.

Good 17" monitors:
Eizo F55S, F56, T57S
miro D1795 T or F (both for Germany only)
Idek Iiyama S701GT, A701GT
Samsung 700p or 700p plus

Good 19" monitors:
Eizo F67
miro D1995 FE (for Germany only)
Elsa Ecomo 19H98
Idek Iiyama S901GT

Of course other monitors can be good as well, but I'm definitely not going to recommend hardware which I don't know.