So I currently have a this video card : http://www.newegg.com/Product/Product.aspx?Item=N82E168... (SAPPHIRE TOXIC 100282TXSR Radeon HD 5850)
and was wondering....My friend is offering me an old ATI Radeon 2900xt 512mb 512 bit interface, would these 2 cards work together in a crossfire setup?
No I don't think so. For crossfire to work optimally (assuming you have a good psu), you'd need to get another 5850 with the same amount of RAM and speed, otherwise, it'll clock down to the lesser. It's not brand specific, but aside from brand, specs should be the same.
The xfx card you linked has a memory clock of 4.0 ghz, while the card you have has 4.5 ghz. Most likely what will happen is the card you have now will run slower at the 4.0 speed if you put the xfx card in, but I doubt you'll even notice a difference.
you can indeed combine those 2 5850 without a problem i have running different types of 4XX0 cards together so make sure to use the fastest card your primary by putting it in the pci-e slot closest to your cpu (which should be slot 0 in general) "ps computers count from 0 upwards"