US STOCKS-S&P 500 gains for an 8th day, boosted by Citigroup

* Dow, S&P 500 end at record highs for third day* Citigroup shares climb after earnings* Empire State manufacturing index up, but retail sales soft* Trading volume lowest of any full trading day this year* Dow up 0.1 pct, S&P 500 up 0.1 pct, Nasdaq up 0.2 pctBy Caroline ValetkevitchNEW...US STOCKS-S&P 500 gains for an 8th day, boosted by Citigroup