You have 1Gb of RAM (random access memory) ... This is normal memory that is used by xp
AND you have 2gb of memory????? LOL
A lot of people get the hard disk size mixed up with RAM and in some cases ROM
Your storage space is defined by how large your Hard Disk drive is ... Even on an old XP machine it is likely to be many 10's of Megabytes (MB) or possibly be Gigabytes (GB).
Volatile memory (that loses it's contents once power is removed) could be 1 or 2 Gb, but it's far more likely to be 2 Mb (mega bytes)
ROM has some basic code (read only memory) and as the name suggests is read at startup of the PC . This is known as non volatile RAM, so the contents stay when power is removed.
A bus is the circuitry that allows all of this data to flow between the CPU,memory and hard disk.
Have a little snoop via google about the stuff above and it will lead to lots of information .... Can't help you on a specific website or book.
As far as upgrading goes .... Yes it can be done, but it depends on the motherboard and what additional memory slots are avialable. Some boards can only take two memory DIMMS (memory sticks), and only of a certain size.
The same goes for faster CPU's . A larger hard disk is nearly always possible, but this will just increase your storage space (like having a bigger shed than your current shed), not give you a performance boost.