A motherboard having two PCI-E slots doesn't instantly mean you can have SLI, in most cases, it means you have CrossFireX compatibility. Like I said, only if the motherboard has a NVIDIA chipset will it run SLI, you could have four PCI-E slots, but unless it's an NVIDIA chipset you can only run a single NVIDIA card.
And no, you can only SLI the same type of card. You can only have 2x or 3x 9800GTX, or 2x GTX 280, etc, you can't mix a GTX 280 and a 9800GX2 into SLI - it won't work.
Do you happen to know what motherboard you have? That way we can verify if you have SLI or not, my second computer has three PCI-E slots - it's a Gigabyte P45 board. But it's not an NVIDIA board, so even though it has 3 PCI-E slots it can't run SLI, only a single GPU.