There are easily mistaken interpretations about the configuration of video cards in SLI/Crossfire set ups. This is my very, very simple explanation of some of the aspects but Google would be a better friend.
One video card (2 GB VRAM) placed in the first, (fastest slot) connected to the monitor/s.
Second card (1Gb Vram) in the next slot, cards joined with a bridge, First card becomes the Master still connected to the monitor/s (through the 2GB capacity of memory) The second cards 1GB of memory is not added to the 2GB. If you reverse the order of the cards, you end up with 1GB capacity of memory feeding the monitor/s..
You can mix different brands of AMD or
Nvidia cards but must be in the same family, slightly different rules for each manufacturer, and CrossfireX different again and many more rules for other components of a video card.
Unless a dedicated gamer, I think it is better to go for the best card you can afford. With duel, triple configuration, you usually need a larger capacity PSU, much better case cooling as you generate heaps more heat, often very noisy, costs more money. I would rather have the latest $400 card than 2 $200 cards. CrossfireX is the exception as the card required is often cheap and an original good PSU may not have to be upgraded.
I was going to get the 64 Bit Professional version of Windows 7
Good choice. I am sticking with 7 as well, for the moment.
I was looking at the AMD Radeon HD 6950 2GB 16X PCIe Video Card or the NVIDIA GeForce GTX 560 Ti 2GB 16X PCIe
The HD 6950 is a disontuinued card, good performance, pricey even second hand. Cards to consider, http://www.newegg.co...2^14-130-812-TShttp://www.superbiiz...EV-G570_2G&c=CJ