I've always thought the 9900/9930 was the best BlackBerry ever. Bigger than your Curve, but slim and with a spectacular keyboard. In between your Curve and the large format 99xx is the 9700. Also a great model but with a keyboard more like your current device. Both are considerable steps up from the 8320. 9900 9700
Best bet is to go to a T-Mobile store and see if the keyboards are acceptable to you. Everybody has different tastes and the keyboard is a very subjective call.