Doing two monitors is pretty simple to do and most any video card today supports this feature. Doing more than two, however, requires a little extra planning.
I don't really know of a single Pavilion system out there that supports dual PCI-Express X16 graphics slots and supports either crossfire or SLI. This means you are forced to getting a single graphics card with more than two external connectors which can all run simultaneously. This means you need to look at the newer ATI 5XXX series video cards which support Eyefinity for multiple monitor setups. You can usually find these cards with a single DVI and two display ports, or two DVI and single HDMI, etc. to give you three total monitors. A base model card to support this feature is the
XFX ATI Radeon 5670 which can run Eyefinity for less than $100. If you can spend a little more and your Pavilion has a spare PCI-Express 6-pin power connector you can step up to the
HIS Radeon 5750 for a little more performance at about $130.
nVidia unfortunately does not have as many options yet to support multiple monitors. I believe some of the newer Quadro FX series cards do support more than two monitors, but those cards are going to easily be more than $150.