System specs of the computer in question, Product Specifications Compaq Presario SR1130NX Desktop PC | HP® Support
There are no Vista drivers, XP only. Also according to the specs it is limited to 1 GB of ram.
As far as adding a video card, I would recommend against it unless you can find a very cheap AGP card that has at least Vista drivers. Then again unless it is less that $20-30 it isn't worth doing as (sad to say) the card will end up costing more than the computer is worth.
In this instance you would be better served sticking with XP until such a time as you can buy a new(er) computer.