Looking at HP support site for that model shows that it only has one PCI slot and the maximum supported memory amount is 1 GB of RAM (which is already installed). So there really doesn't seem to be much in a way to upgrade that computer you have I'm afraid.
Here is a link to the HP site for your model: http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=28717&tmp_task=prodinfoCategory&cc=us&dlc=en&lc=en&product=1844325.
You'll have to start looking for a whole new computer if you want/need a upgrade.