With XP Home you cannot allocate more than 2GB RAM to applications - you can allocate 3GB with XP PRO, however.
See this
MS article.
Personally I regard 2GB as a practical max (certainly for XP home). Add more if you wish, but it will be of questionable value.
Read the mobo manual for how to add RAM - it may be that you can add RAM in pairs to improve performance, but it is probably not necessary, so you can have an odd number of DIMMS, or DIMMS of different sizes.
You should accept the page file size Windows recommends - unless you have good cause not to. What's 6GB out of 320? For performance, don't put it on the system partition, don't spread it over multiple disks and do make it a fixed size (to help avoid fragmentation).
SD