Matt, I'm assuming that you're using this in the office and not in the field (with Site Mate and hooking up to GPS). As far as processor speed and amount of memory and such, any laptop that you buy today will be more than adequate for the software. However, two things to be sure you have:
1. A PC Card for reading yield cards. 2. A CDR/W for making backups. With yield data, your backups will get very large and it's very helpful to have a CD R/W for backing up to. If you can't get this on the laptop, the other option would be a USB Drive.
If you will be wanting to hook GPS up to the laptop (such as with Site Mate), you will probably want to try to find a laptop with a serial port (although they're getting harder to find).