All of my cards are nVidia, and they mine quite well, despite Ignatius' comments.  However, Ignatius is very correct about CPU mining, and anything older than the 700 series GTX cards may not do so well (so no, a 500-series card is not very great).  The best place to get the latest version of CCminer is from the CUDAmining website, http://cudamining.cc/url/releases  There are also some tutorials there (CCminer is actually much easier to use than most miners geared for AMD cards).  Also, the main CUDAmining thread is here: https://bitcointalk.org/index.php?topic=167229.0  I also strongly recommend aiming your nVidia card(s) at any X-based coin or pool (X11, X13, etc) as they perform very well on those algorithms (also amazing with the M7 algorithm, using DJM's latest version of CCminer).

On a side note, the latest 900-series GTX cards outperform equivalent AMD cards in hashpower, and use about a quarter of the electricity.