3-Way SLI is only suited for the guy who has to have the very greatest at the very moment. You don't seem like that kind of person, since you only upgrade every 5 years. You're also going to need a 1.2KW PSU and you're going to have to sacrifice space for a nice sound card. Also, an 8800 GT is also not currently supported in nvidia's 3-way SLI plans (unless theres some news I havent heard yet). 
  
What you should be concerned about is making sure there's support for Nvidia's next high end card, since one or two of those may blow away 3-Way SLI.