The Saphire 4850 is a single slot card! If I had to choose a card out of the two it would be the 9800 GTX, but I would wait for a better card to come out from Nvidia to beat the 4850's price and performance. I wouldn't settle for second best (the 9800 GTX in your case).
This reminds me of when the 3870X2 came out, it was the best card out, but than came along the 9800GX2. I'm glad I waited a while back than, and wow it was a short wait.