32-bit Windows allows a maximum of 4GB of system memory. This includes both system RAM and video card ram (VRAM). So if you have a video card with 1GB of RAM (most do nowadays), then that limits the total usable system RAM to 3GB (the 1GB VRAM is taken out of the 4GB total). Even if you have 8GB installed, the OS can only support 4GB total. An exception is Windows 7 Starter, which only supports 2GB.
64-bit Windows allows a maximum of 192GB of system memory. This is lots, obviously. The most supported by any modern day motherboard at the highest end is 48GB anyway. Exceptions to this are Windows 7 Home Basic which has a max of 8GB, and Windows 7 Home Premium which has a max of 16GB, which leaves Windows 7 Ultimate, Professional and Enterprise with 192GB.