If you are using Windows Vista, the video subsystem only allows drivers from one company e.g. Nvidia, ATI, Nvidia etc.
Therefore, you should be able to get this working with any second Nvidia card.
SLI however is dependent on another video card in the same series, so if you get a $50 card, do not set it as primary (If you motherboard has one slower SLI slot, put it in the slower slot).
If you are using Windows 7, you should be able to have a secondary card of any manufacturer (that being said, I still prefer to keep it within the same company!).
To sum up - using a different card will achieve what you want without problems, however use as secondary otherwise performance may go down hill.
If you have any other outputs such as VGA or HDMI, you may want to try and get an adapter to power all from the first card.