I implement a large number of call centers. Since Avaya is moving from IP Agent to one-X Agent I would recommend staying on hardphones (I've played with the ROI numbers). You can use the 96xx series phones. We have customers who use the 9620, 9630, and 9640 (I would stay away from the 9650 for agents). The 9620 is minimal. The 9630 is gray scale and the 9640 is color display. They all support headsets.
As noted Avaya has just released some new phones. I really like the 9611. Of course it is new so there won't be much of a track record. Look closely at the pricing. For instance if you have non-call center users or managers and you want to give them blue-tooth capabilities the 9670 is only a few dollars more than the 9640 with Bluetooth adapter and the new stand required to fit around the bluetooth adapter (poor design).
I have stayed away from the 16xx series phones like the plague. I can't justify going back to hard labels. Unless you have third party SIP endpoints you need to deploy why add the complexity of SIP endpoints? You might want to convert to SIP in 2 years if you migrate to NGCC but for now H.323 provides a simpler deployment for agents.