<--my level may label me as a stranger or newbie but my knowledge of this graphics card is vast as i've owned and researched it since the day it was released. wow, i had to register just to respond to this post. I'm appalled at the answers to your question i've seen in this thread and the lack of knowledge and information that some of the subscribers gave you.
1. The 7950gx2 is essentially 2 7900gt 512mb's with an internal sli bridge.
2. You DO NOT need an sli motherboard to run this card but your current motherboard should be on the supported list found here:
http://www.nvidia.com/content/geforce_gx2_sbios/us.asp
3. It does get hot, proper case cooling is a must, a side intake fan or a 120mm front intake fan will help to cool this card, but also keep in mind that these cards are meant to run hot.
4. The card only requires (1) ONE pci-e power connector (or a 2 molex to pci-e adaptor) , it can be run off a good quality 450W power supply w/ 26A on the 12V rail. (This is what I'm using to power it - OCZ Modstream 450W)
5. Try to talk your friend down about $25 and you will certainly NOT regret your purchase, it is a fantastic card for playing all modern games at high resolution and is easily overclockable.
6. The 7950gt is not a faster or better card and the 8800gts 320mb is fast but lacking crucial mb's of ram to run higher resolutions w/ aa turned up. (see many reviews)