Let's take a look at Table 1 ASHRAE 2008 Thermal Guidelines for Data Centres

Based on this table, I would say ranging from 18°Cdb to 27°Cwb and 20% to 80% Relative Humidity (RH) for the most stringent environmentally controlled data centres, this is not much of guidance, isn't it?
Now, let's look at humidity first, the key to relative humidity selection is really ‘non-condensing‘, which effectively meant control of the maximum allowable dew point temperature within the conditioned space! So as long as you can ensure no condensation will occur inside your data center, you will satisfy your equipment supply vendor's criteria on relative humidity. In actual fact, I would prefer to specify dew point temperature rather than relative humidity.
The level of temperature and relative humidity control will depend on your data centre class, here is another guideline from ASHRAE for information.

Even back in 2011, my internal design temperature criteria were 27°Cdb, using indirect heat mass exchanger or some would fondly call it indirect “swamp coolers”.
Why 27°Cdb?
- is because it's still within the guideline provided by ASHRAE
- is if you look at equipment specifications on operating conditions, you will see that most of these modern servers, switches and etc have maximum operating temperatures around 40°C and some even at 45°C.