Some people will advise you to stick with the stock 186 deg, personally I would go with the 160 deg thermostat and that's what I did on my car when I replaced the water pump (with OEM pump). I also installed the 3rd radiator and after both of those changes my engine has run noticeably cooler while still warming up generally just as fast as before and still easily getting up to full operating temp everytime I drive it. Hartech has a good writeup on why a low temp thermostat is a good idea, among other things... read
PART 5 of their buyers guide here:
Hartech | Porsche boxster engine repairs, Porsche engine repair, Porsche 996 engine repairs, Porsche engine problems
BTW, you have an
2005 99
6?