R,
the design of a site (number of TRX, number of sectors) based on voice traffic + data traffic is the right way to go.
Unfortunately it is not easy to predict how much and when the GPRS is going to be used. It’s also difficult to estimate the amount of throughput required by the users in order for them to feel it’s “good”.
For instance, for web surf, you don’t need so much throughput. But what you need is fast response time…
an easy way is to always ensure the voice TCH are not using up all the timeslots (at least 2 or 3 are always available per cell, in order to provide “satisfying” GPRS access)
QoS indicators can help you with that. Then you can perform drivetests in order to check out the overall data service “quality” of your operator. Do you “feel” that’s good enough ? Don’t forget, we’re talking about a very subjective matter here. You need to isolate some key performance results in order to qualify your network. Ping response time, throughput, reselection time, retransmissions, etc…