Best Buy is not a participating member in the MSN eShops Holiday Reward rebate program so it would not work as described. Anyway if it did, the math is still wrong. $400 - $80 MSN Rebate (4x $20 for each $100) - $50 Compaq rebate = $270.
You could buy from RS @ $499.99 and then try to PM with Best Buy and then do a AMEX BVG for another $100, buy RS does not PM other retailers ot etailers.