Thanks. I was hoping someone would confirm I wasn't being daft.
I've managed to get 4GB of RAM onto the PC now, (2 x 2GB DDR2 which should be fine) but oddly the Asus P5W DH Deluxe motherboard, is only allocating 3200MB of that to the OS (in the BIOS which is current, so it's not windows doing something weird) and the BIOS suggests that something is "appropriating" the rest

Still it's probablynot a biggie since Windows 2003 can only use 4GB RAM at best.