my recommendation on crossfire or sli would be to get a better video card and then upgrade it in a few years when there are better out. this will use less power and be cheaper in the long run rather then trying to new video cards.
Yes but sometimes if the person don't have too much money he can buy two appropriate cost graphic card and make a crossfire thus will have the same time as the expensive one.
I will recommend you to buy two low cost card if you don't have too much money but if you have enough then buy an ATI Radeon HD 4850
http://www.anandtech.com/video/showdoc.aspx?i=3338
http://www.tomshardware.com/news/ATI-Radeon-4850,5705.html
Or an NVIDIA 8800GT
http://www.nvidia.com/object/geforce_8800gt.html