You need to add the /3GB switch to your boot.ini.
/3GB
This switch forces x86-based systems to allocate 3 GB of virtual address space to programs and 1 GB to the kernel and to executive components. A program must be designed to take advantage of the additional memory address space. With this switch, user mode programs can access 3 GB of memory instead of the usual 2 GB that Windows allocates to user mode programs. The switch moves the starting point of kernel memory to 3 GB. Some configurations of Microsoft Exchange Server 2003 and Microsoft Windows Server 2003 may require this switch.
This guy put together a fairly good list... of things it might break or what to expect.
http://www.gehrytechnologies.com/cat...rge_memory.htm