My P5K deluxe sensor reads around 38C in an Antec Sonata 1.0 which
doesn't have the best airflow so it runs on the hot side.
I'd look at changing your cables and maybe increasing the case airflow by speeding
up an intake or exhaust fan, adding another fan for better airflow,
using a PCI-slot-cooler fan or whatever to get more hot air out of the case.
I don't think 48C is a horrible motherboard temperature at maximum PC
load in a warm room, but I think it is on the high side and can probably
be improved through better airflow if you have a case that's roomy enough
to get things a little more openly ventillated.
If your CPU's staying cool under max load and the system is stable I guess
it's all "OK" but "could be better".