Most copies of XP are 32-bit, but there was a 64-bit flavour (only of Pro, I think) made.
A computer running a 32-bit operating system can only use the first ~3GB of the memory you have installed. So installing more than 3GB of RAM in a computer with a 32-bit OS is a waste of money, although it shouldn't actually hurt anything.
(Firefox really does balloon up if you have enough tabs/windows open for long enough, but I notice it a lot less since I upper the RAM in this thing to 16GB. Now I can run a week-old Firefox session and multiple virtual machines without processes getting unexpectedly terminated on me.)
A computer running a 32-bit operating system can only use the first ~3GB of the memory you have installed. So installing more than 3GB of RAM in a computer with a 32-bit OS is a waste of money, although it shouldn't actually hurt anything.
(Firefox really does balloon up if you have enough tabs/windows open for long enough, but I notice it a lot less since I upper the RAM in this thing to 16GB. Now I can run a week-old Firefox session and multiple virtual machines without processes getting unexpectedly terminated on me.)