I am a sales manager. I have found GTD, and in particular, the Outlook plugin, really helpful in staying on top of things. I have set up my @ folders so they are unique to my requirements - I don't use @computer, @phone, and things like that.
However, we also use a CRM program (customer relationship management) at my company. I make sure I log all correspondance with customers and prospects in this database. I manage my tasks and projects using GTD. However, because we have so many sales leads and opportunities to track, I don't make each of them a GTD project, even though they really are - that is what CRM is for.
If you do not need to use a centralized database in your sales, then I would highly recommend that you look at a free account with Salesforce.com. You can sync everything up with Outlook, and it will blow ACT away. Some features may not be available in their free version, but if I didn't have to use a centralized database, I'd even pay for a single account with Salesforce.
Good luck!