This is really a general question mate.
I realize you are using max so we will leave this one here for now.
please direct any future content thats general to the geek culture area.
I run maya on a PPC using a 19inch CRT and a 17 inch LCD.
The reason why i have both is I also do print / web and video, and require both for these elements.
To future proof your purchases, save desk real estate and to probably save a bit of money, conisider a larger one for your modelling and a smaller one for tools.
In all fairness though this is really a financial question only you can ask yourself.
If we all had the cash we would have 3 x 30 inch wide screens attached