I say SLI isn't a waste of money if you already have a card. But buying two cards for SLI at one time is a waste. SLI 7800 GTs would out perform a 7900 GTX and a X1900 XTX. Don't even bother with trying to sell your card.
Really you should just stick with the 7800 GT and wait till the R600 or G80 comes out. But if you want performance now, I say grab another 7800 GT and SLI them. I am usually against SLI, but since you already have one of the cards and SLI is starting to mature, along with games using SLI more efficient then before I think SLI isn't that bad. I think it is stupid to buy two cards in one upgrade and SLI them, when usually a single card will play games fine.
So either wait till the R600/G80 comes out or get another 7800 GT and SLI them. The boost of performance from a 7800 GT to a 7900 GT isn't worth paying 300 dollars for.