SLI gets you another 20-25% performance boost. You don't get twice the graphics bump by using two cards. You're better off getting one powerful card that has the ability to do SLI or Crossfire so in 3 or 4 years when you're ready to upgrade a bit you can buy another of the same card at a much reduced rate (as it's now 4 years old tech) and SLI it then if you so choose.
A few additional thoughts:
Don't scrimp on the power supply. Buy a slightly less powerful graphics card if you need to save a couple bucks but get a good power supply. Modularity is awesome in power supplies.
Stock coolers will be loud.
Don't use RAID0 for data drives.
Memory is cheap. Buy a lot of it.
SSD drives are awesome to boot from but expensive to play from (at least currently).
Onboard sound is just fine for 99 44/100% of the people.
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