Armour Residential REIT (ARR) could be a solid addition to your portfolio given its recent upgrade to a Zacks Rank #2 (Buy). This upgrade primarily reflects an upward trend in earnings estimates, which is one of the most powerful forces impacting stock prices.A company's changing earnings picture is at the core of the Zacks rating. The system tracks the Zacks Consensus Estimate -- the consensus measure of EPS estimates from the sell-side analysts covering the stock -- for the current and following years.Sin ...