Windows will still only use a maximum of 2gb per app though, and only utilise a maximum of 3.5gb
you need to also set the /3GB flag in your boot.ini
Thanks guys! 
Think I'll stick to getting 1Gb. However, the stick I have in now is 667Mhz, so as the mobo can take up to 800, it may be worth getting a new 2Gb 800 stick, as I don't think it's good to have two different sticks on the board.
Job done.
Just bought a Kingston 2Gb PC2 6400 800 Hyper stick for £19.54 + vat/delivery.
Ouch, thats expensive!
32bit Windows can only access 4G RAM (ignoring server style PAE extensions not supported on desktop), and internally, the NT kernel allocates 2G to kernel, and 2G to each application space.
As PC architecture needs memory holes, really only about 3-3.5G is usable, so 4G not worth the effort. 64bit Windows overcomes this, but introduces a whole load of other hurdles).
As the kernel uses bugger all, the boot.ini mod suggested by PaulW is worthwhile on machines with more than 2.5Gb