While they both run the most recent software release, the 998 has dual processors (faster map redraws and screen capture writes to SD card), an ethernet port for future ethernet networking, and also a video out jack to record or display the unit’s output on another monitor. The faster GPS response rate that Wade mentioned is actually common to both the 997 and the 998, as long as the 997 has been updated with the most recent software.
You’re going to have a hard time finding a new 997, as most retailers were able to blow them out for about the same cost as the new 898. If you can find a 997, you’re getting a LOT of technology for the $$. However, like Wade, I strongly suspect you’re heading for a new 998 instead.