Well, if SLI doesn't allow for more than 2 cards, getting a board with 3 slots means you connect only 2 via SLI and the third just rund on it's own calculating physics, if that is even supported yet. I do know it hads been announced though and there has to be a reason some boards have 3 PCIe X16 slots.

And concerning your question, no, not necessarily. If the game is not even optimized for dual cores, having four or eight cores won't help you in any way. I play Medieval 2 on highest settings and huge units and with more than about 5000 men it lags but my CPU is not at 100%.

Concerning the cost, I recommend you go to a local shop and ask, might be hard to get as a complete package and to be honest, I wouldn't even buy the stuff if I had the money since it may take a year or so and it's almost outdated again. If you get a cheaper still powerful computer now, you can get a new one in a year or two and still pay less.