There have been so many threads with the same questions about SLI and so many wrong answers given to people that maybe a mod should sticky this.
To use SLI you need 2 cards with the same GPU i.e. 2 8800GTs, 2 7800GTXs, etc. They need to have the same amount of memory. There used to be a way to SLI 2 cards with different amounts of memory using RivaTuner or some tool of the sort but I'm not sure that's still possible.
The GPU's DO NOT need to be the same speed on both cards nor from the same vendor, just the same GPU. An EVGA 8800GT 512MB will work just fine with a BFG 8800GT OC card.
The memory DOES NOT need to be the same speed or same "timings" on both cards, just the same amount i.e both cards have 512MB. The memory is not "shared", if you have 2 cards with 512MB each you do not effectively have 1GB of video memory. Each card uses its own memory exclusively leaving you with an effective 512MB of video memory.
If one card has higher clocks than the other card it WILL NOT be automatically underclocked to match the speeds of the slower card. On every mother board I've tested this on it has worked that which ever card is in the primary pci-e slot (usually the top) the other cards clock speeds are set to match that cards speeds, so:
1. Higher clocked card in primary slot, slower card is automatically overclocked
2. Lower clocked card in primary slot, faster card is automatically underclocked
This applies to memory and GPU speeds, I haven't checked the shader clocks but I'm guessing they would sync up as well, maybe someone can verify this or I will when I get around to getting my 2nd 8800GT.
MSI MPower Z87 | 4770k | CNPS9900MAX-B | 8GB G.Skill 1866 | EVGA GTX760 |ThermalTake 850w | X-Fi Titanium | HAF | Asus PA248Q | Lycosa | Death Adder | Win8 Pro 64
MSI GD65 | 2500k | H50 | 8GB G.skill | GTX570 | Ubuntu 13.10
17" Inspiron | i5 | 4GB DDR3 | 500GB HDD | Win7 Pro 64