While you are entitled to your reasons for insisting on an SLI based system, the fact is, it has been proven time and time again to be cost ineffective and borderline impractical for mainstream gaming and even some mid-high end gaming rigs.
Also, this route limits your options (good or bad) since the technology is not *officially* supported by arguably the most popular C2D chipsets out there namely the 975 & 965 series. Although there is now the nForce 680i SLI from Nvidia that edges out both of the Intel chipsets mentioned earlier, but it comes at a pretty HEFTY price for most gaming enthusiasts before even factoring in the SLI cards.
If I were you, I would go the single graphics card route and with your budget, I would say the P5W DH Deluxe from ASUS or if you don't have to have a 975X to go with the P5B Deluxe also from Asus. It’s far less expensive compared to the 975X P5W DH Deluxe motherboard, but it does feature the ICH8R Southbridge and consumes less power compared to the 975X, and performance wise, it will give roughly 95-100% of the performance of the 975X without overclocking.
And while it may not be the best board in terms of price vs performance vs features vs overclockability, it’s above average in all of them, which IMO is a good thing overall.
Lastly if you do not even mind setting your sights much lower, there is the GA-965P-DS3 & GA-965P-S3 from Gigabyte.
Goodluck!