MS SQL Server Max Memory Size for given Server Physical Memory

Glenn Berry, over at SQL Server Central, has written an excellent and straight-forward guide to initial settings for max memory in SQL Server, given the amount of memory available on the server.

Suggested Max Memory Settings for SQL Server 2005/2008

I would encourage you to visit his article for details, but here follows the crux of the matter:
Physical RAM                        MaxServerMem Setting
2GB                                           1500
4GB                                           3200
6GB                                           4800
8GB                                           6400
12GB                                         10000
16GB                                         13500
24GB                                         21500
32GB                                         29000
48GB                                         44000
64GB                                         60000
72GB                                         68000
96GB                                         92000
128GB                                       124000

Again, note that he goes on to give more advice on tuning with additional products, and how much memory should be available to the system when under max load. I would encourage you to check out the original article.