That depends on the devices you have plugged into your computer. Like a video adapter might map around 300 MB to it. Other PCI cards get some. The consumers can be found in the device manager (View -> Resourced by type, Memory). So you can only get more memory by not having a good video card, which is not fun. There is no 32-bit Windows Server anymore, so that is not a possible path. It is either Windows NT 6.1 x64, or, Server 2003/2008 with PAE (not R2 but "Vista").