|If its 32 bit XP Pro your computer will only recognize 3.5 GB.|
That is entirely dependent on the motherboard. With some motherboards it can be as low as 2.8 Gbs
You are also making the assumption that the motherboard can take 2Gb memory modules. A dangerous assumption to make. 512 Mbs modules may well be the maximum the motherboard can take, Even if the motherboard could take 2GB memory modules, there is a maximum the motherboard can support regardless of how much memory you can physically install.
As JohnR said, you need to know the motherboard details before anything can be said about memory.