(1) Your temps are close to nornmal for (A) ambient temp of 78 F - This time of the year most have an ambient room temp of 68->72 F. ( for ever deg F rise in room temp the cpu temp will go up 1C->1.5C ) (B) using the Stock Intel HSF - One of the poorest performing HSF, aftermarket much better. (C) playing BF3 - ?? how long into game play.
(2) Max temp, before CPU starts throttling back is mid 90s. Recommended is to keep below 70 C ( My self imposed limit is 65C
(3) While the Stock HSF is OK for Game play it does not allow (not recommend) for OC ( the I5-2500 K is very easily OCed to 4.2->4.6). Nor is it adequate for runing Prime 95 for extened time to verify System stability - may reach 70->80 C with in a short time.
... And I always recommend runing prime 95 for min of 4->6 hours to verify all new systems and/or when ever memory is changed (or settings) and when OCing.
.. Most recommended 3rd party HSF based on cost/performance is the Hyper 212 (ball park - $30), Myself I like the higher end ones - I have the Zalman 9900Max - But it is not cost effective unless ON SALE. It is also hard to mount in that it requires eccessive pressure - But I do like. Remember, while your temps are fine - the cooler the better.