Database question for upcoming project

We will be developing an RIA that will have a lot of database access.  Think something like a QuickBooks but with about 50 transactions entered per hour per user.  Users will be in the system for 7 to 9 hours a day and there will be around 20,000 users, all logged in at the same time.

Reporting will be done just like a QuickBooks style app plus a lot of extra things you don't do in QuickBooks.

Our operations is familiar with W2003 Server and MS SQL Server so they are recommending we stick with that.  I originally requested Linux and PostgreSQL.

How far can a single database server get me?  If we have a 4 processor, 8 core, 128gb server, how far am I going to get before I need to shard or do something else?  I know there are a lot of factors involved but in general for this size of a site, what should the strategy be?  

I've read almost all articles on this website but most of the applications are not RIA type of apps with this type of usage or they are architectures for sites with millions of users which we also won't have.