8GB Ram is completely overkill for most pc users. NTM, if the Pavilion came with XP, then it may not even support over 4GB. For XP, 2-3GB will be plenty.
Framebuffer on a video card should not be used as a determining factor when buying, unless you have specific goals... like running GTAIV with 16xAF and FSAA.
A high end video card will Not run in a HP without a powersupply upgrade. IF the computer has a PCIe x16 slot, then I wouldn't put anything over a x6xx series ATi card or x600gt nVidia card in. Worse luck if there is an AGP slot, or no slot at all.