Can I use 2 graphics cards

Answering your questions.

1. GPU Memory is determined by the memory of your Graphics Card, system main GPU memory is determined by your primary graphics card. It's not capped by the motherboard.

2. SLI or Crossfire is good if you want to do the investment by parts, but i personally do not recommend because SLI or Crossfire doesn't give near double performance. If you have the budget go on, but if its not the case you can buy a powerful single card and you will be ok. Using two of the best graphics cards may require you changing your power supply.

3. Transfer rate it wont be a problem except of those cards that come with two GPU on a single board like AMD Radeon 7990 or Nvidia GTX 690, those cards may cap out the entire bandwitch of PCI Express bus connector.

4. If your power supply only have two six-pin pci-express power connectors you may use two mid-end video cards like GTX650, or one high-end video cards that may use up-to two six pin connectors, consider that other video cards may use one six pin connector and one eight pin connector. Obviusly this depends of the video card you want to put on your system.

Depending of what use are you going to do of your system, it is depending your inversion. You may edit your question giving more details for a better answer.