wbino,
Your refresh rate shouldn't affect your FR as far as I know - generally speaking, however, a higher refresh rate is much easier on your eyes. I run all my monitors on the highest refresh rate possible, and I don't have any issues.
Like I said, simply play around with the settings, and avoid running programs in the background that you don't need (check your task-manager, but don't just close a bunch of programs that you aren't sure about. If you want to know what a process is, just google the name to learn about it).
I agree, you probably don't need to bother getting another 8800 GTX for SLI, especially for SINS which is a relatively low-requ game. If you play lots of FPS and such, then go nuts, but I agree that a better investment would be a new monitor. However, if you go much bigger than 22", make sure you have adequate video memory to use it. 256 MB is good for anything less than about 24" or so, if you go bigger, make sure you have at least 512 mb.
You're the only one who can decide what you like best for settings. Simply play with the settings, and choose what you like best. The biggest FR hits usually come from things like shadows, AA, and bloom effects. Just play with the settings, and if you are going to invest any money, I really agree with Annatar to get a new monitor before you bother with SLI. Don't forget, not all games support SLI, so you have to be careful when you invest that kind of money - make sure the games you play or are going to buy support SLI (if and when you get it).