unless you've found a way around the Laws of Physics Win XP 32 bit will only ever recognize 4GB RAM -- Maximum.
From this you need to subtract amounts used by the video if your video is integrated with the mobo.
The maximum amount the OS will probably report as available will be around 3.3GB.
W2K3 server X32 R2 Enterprise edition with SP2 applied can address 128 GB RAM BTW.
W2K3 Server R2 Datacenter edition X32 can address 128 GB
It does this by using the /PAE feature as explained in this article Large memory support is available in Windows Server 2003 and in Windows 2000