I have a system with the following setup : AMD 955BE CPU, 4GB RAM, 850W PSU and an ASRock M3A785GXH/128M motherboard that has 3 PCIe slots.
One is 16x, the other is 8x and the last is 4x.
If I were to do a 2-way SLI setup with a GTX 660 (using the x16 and x8 slots), would the 8x slot impact performance?
For the moment I'm using a 1680x1050 monitor and in the future I am not planning on going any higher than 1920x1080.
There are some benefits to using 16x slots on the motherboard for multi-GPU setups, but the difference is only a couple of percent. You might see a 2-3% increase in performance using 16x/16x, but it's not going to noticeable for multiple reasons.
First, 1680x1050 is going to be a huge limitation for any multi-GPU setup, and secondly, your CPU will be a limitation in any multi-GPU setup. I would go ahead with the 8x/8x SLI solution and not worry about it, you'll be fine.