Yes you're right about the memory speed. You've taken yourself out of dual channel territory. You might want to upgrade your motherboard before the memory, because memory differences often make you repurchase. Since you already have an 8800 GTX, going SLI will only gain you about a 30% improvement if the most optomistic reports are true.
Computer speed is kind of like seeing: in order to see a difference in brightness one object must be double the brightness of the other. A 30% improvement would look good in the numbers but would feel like nothing much changed. If you're concerned about a hatful of numbers, upgrading is great. If you want your computer to work noticeably better, it's a good time to circle the wagons, save your money and wait for the next quantum leap. It won't be long.
