5.5 gig? What size sticks and how do you have them installed?
What are you doing that you need so much RAM? 4 gig is plenty enough for all but the most demanding users running XP.
I see the post above, but seriously, a Ramdisk? This is an ancient technique from days gone by when RAM was expensive.
I would not buy XP64, not a lot of support for it, and it is outdated.
If you want a new 64 bit OS, go straight for Windows 7.
Go here download and run this, it will tell you of any compatibilty problems.
http://www.microsoft.com/downloads/details.aspx?familyid=1B544E90-7659-4BD9-9E51-2497C146AF15&displaylang=en
If you can run a 64 bit OS, you can download the pre-release version of Windows 7, or just wait for the official release in a few months.
Personally, I would remove all but 4 gig, preferably 2x2 gig, and you will be just fine running XP. There is no reason to want to more, since the OS can't use it all, and you most likely won't actually need more anyway, especially with a Celeron system.
More memory would be the least of your bottleneck worries, the very, very least.