Ok, i will answer your questions in the order asked:
1. yes, you need one that supports SLI or crossfire (SLI is nvidia, crossfire is ATi). It will have 2 PCI-e slots if it can handle it, and you will be able to find out if it is nvidia or ati from a google search.
2. The cards need to be of the same chip, so for example 2 9800gt's would work. They don't have to be the same manufacturer, model anything, just the same chip, and remember that a 9800gt is the same as an 8800gt. Also bare in mind that if one card has 512mb of memory and the other 1gb for example, both will rn at 512mb, it iwll go to the lowest memory, the same with clock speed, it will sit on the lowest for both cards.
3. So long as the motherboard and power supply support it, you will be good to do it
4. Yes, it is faster than 1 video card. Theoretically, it is double the power, but it is always slightly below, but it varies from marginally less that double to quite a bit less.
5. No, not always. Some applications it does work with even without the dual card option, but some it doesn't. Check with the game if it supports it, again a google search will come up with what you want.
6. You can use more than 1 monitor now if your video card has 2 outputs. If you need more power but can't afford a better card, then it is worth it. Let us say that you have 1 8600gt and you want more but can't afford a 9600gt or 8800gt, you could buy a second 8600gt and get twice the power for much less.
7. see 4.
8. Yes, it works on vista.
What you should remember though aswell is that you don't just need the cards, you need an sli or crossfire cable, it is a cable that links the two cards. They are only cheap, but you need one, otherwise you have 2 cards running independantly, not together.
Also, It may be cheaper to sell your current card and get a more powerful one. It is almost always better for cost in the short and longrun to get a new single card. This is because you can sell your current card and use the funds from that towards a new one, added to the price you would have payed for a second one, that can often be a nice little bit of money. And remember it isn't just the cost of the card, you ahve to power both and that could mean getting a new PSU, and to power it you will need to pay more, graphics cards are the biggest electricity user in your system
