Memory is usually under a small hatch on the bottom of the computer. You won't do any harm by taking a peek. If a slot is free, add something (search Kingston or Crucial sites for modules compatible with your machine -- probably cheaper than from HP).
If both slots are taken, you'll have to figure out what size the present ones are and whether there is potential to install larger capacity modules.
Found it. There is one stick of 1 GB RAM, and under that is a stick of 2 GB RAM. No other spots for more.
So, is it possioble to put a 2GB stcik in where the 1 GB RAM is to give the machine a total of 4GB RAM?
Yes, the system supports up to 4 GB of RAM. But, depending on the version of your operating system it may not matter. If you are only using a 32 bit version of XP, Vista or 7, then the operating system may not detect anything over 3 GB of RAM (this was/is especially true with XP). However, if you are using a 64-bit version of the operating systems, there will be no issue.
Yes, the processor is 64-bit capable but it's the Operating System that will ultimately determine what we need.
Go to Start > Control Panel > System and Maintenance > System. On that page there should be something that says "system type" and next to that it will say either 32-bit or 64-bit.