Your client could use these variables when deciding the suitability for Ultrapeer mode:

Bandwidth capacity: at least 256kbps in both directions, dedicated only for hosts traffic, for example
Uptime (average, for a week(?) long perioid): Couple of hours minimum
CPU power and RAM: Depends on the optimization of your client
OS type: Win9x nor old MacOS systems should not be promoted to ultrapeer mode. Generally all OSes which lack in TCP performance should remain in Leaf mode.
Horizon: If host fails to maintain proper horizon, it could demote back to leaf mode
Demand for UPs: If most of UPs in the horizon are full, you could be promoted easily to Ultrapeer and vice versa.

There's a couple of suggestions... The more varialbes you take account when deciding the proper mode, the more scalable will your client be when the network grows and the better will the network work. Also, good target would be to maintain the selected mode long enough. The more stable the network is, the better performance will it have.