What is the appropriate horsepower (wattage) for a new pair of PSB T2's?

The main factors in deciding what power you need are the size of your room, type of music and your preferred listening level. Those speakers are fairly sensitive (90dB) so unless you have a particularly large room then you probably don't need anything greater than 50W and can probably get away with a fair bit less.
If you have a digital multimeter and a digital source (DAC etc) then there's an experiment you can do to work out how much power you need at your preferred listening levels. If you're interested I can find a link to the instructions.

There was a poll over on diyaudio some time ago where people measured their maximum power requirements using their own amps and speakers. 71% needed 25W or less with only 16% needing over 100W and that is the maximum power they needed not their normal listening requirements. If you don't mind reading lots of pages about people questioning the maths the thread's here.
If you do go for a lower power amp (and there are good reasons to) then you just need to make sure the designer hasn't skimped on the power supply. I rate mine to deliver a sine wave at full power into the lowest rated impedance so they're easily up to the job... others don't always want to pay for that much copper.