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.
