I'm using a ZyXEL Prestige P660HW-61 - not quite the same model, but pretty close. The spec. I'm seeing is as follows
System Name: P660HW
ZyNOS F/W Version: V3.40(PE.11) | 05/22/2006
DSL FW Version:TI AR7 05.01.03.00
Standard:ADSL2+
(This is obtained via the System Status menu entry under Maintenance once you log on to the router.)
I'm fairly sure that all the ZyXEL P660 models can handle ADSL2+. (When I got my router it had 2005 firmware and the box already said it could do ADSL2+.) Unfortunately there doesn't seem to be any way to force the router to use one mode in preference to another - at least not without going into command line mode. From the Web interface it looks like the router just automatically picks ADSL2+ if it's an available option on the line. That's what happened when I joined the trial.
You're getting a new router, so what the ZyXEL does may not be relevant for long - but you might want to check whether you get similar info from your router before you make the switch.
Graham