I can't help with the rest but this bit -
"Addressing Memory With 4-GB Configurations
Your computer supports a maximum of 4 GB of memory when you use four 1-GB DIMMs. Current operating systems, such as Microsoft® Windows® XP, can use a maximum of 4 GB of address space; however, the amount of memory available to the operating system is less than 4 GB. Certain components within the computer require address space in the 4-GB range. Any address space reserved for these components cannot be used by computer memory."
means that if you put 4Gb ram in it will not all be recognised as any memory on your graphic card or any other cards etc will be deducted from the 4Gb as XP can only 'see' 4Gb in total from all the sources.
most people just put 3Gb in ram as no benefit is gained over this (just my opinion) others may have differing views.