Compaqs generally are not very friendly when it comes to user upgrades. I remember when PCI cards were all the rage and Compaqs did not provide an extra PCI slot. I am assuming they have not improved much since then, and also the Presario is known as a "package" PC- everything is bundled for the convenience of those not familiar with PCs.
My advice, take it to a shop and get them to do it. Of course buy the parts yourself (you'll get them much cheaper), but don't try the installation yourself, because you're going to need not only a new card but a new power supply and more RAM. You don't want to attempt something and have it be worse off than before. Just my suggestion.
