The EP45-DS3R supports neither SLI or DDR3.
It is a DDR2 platform that supports only CrossFire.
That is not all bad as ATI's offerings are extremely sound this time around and DDR2 800 CL4 performs the same as any DDR3.
For the RAM, look into 4Gb of DDR2 800 with 4-4-4-x timings.
Scrap the GTX 280 and look into either a 4870x2 or CrossFire 4850/4870's.