How to break off into my own department

I have worked with a small company for 6 years and brought us from a 2 PC company to 35 PCs, 1 Novell and 1 NT server, several client-server apps, etc. All along, all my expenses, (new PCs, cabling, internet, software) have been coded to accounting. I am working on a project to break myself away from acounting and establish myself as a department with my own budget, etc. I have gathered quite a bit of info from Techrepublic articles but am looking for, perhaps, guidelines on the steps I should take to do this. How do I plan a budget, etc. Any help would be GREATLY appreciated.