So I finally replaced the thermal paste (it was completely shot as suspected) and the CPU's now running at 50-55 (Peaking at 60) while I'm playing Planetside 2. Once I get the Noctua fan I'm sure that'll come down some more, but I'm quite happy with a 40 degree drop .
Going to start looking into overclocking now. How do I go about overclocking the i5 2500-K? How high can I push it without breaking anything?
Glad to hear that it was the paste.
OCing is simple, but it depends on your setup.
Search Google for "overclock i5 2500k msi p67a-c43" and you'll find a plethora of results.
https://www.google.com/search?q=MSI+MS-7673+P67A-C43#q=overclock%20i5%202500k%20msi%20p67a-c43FWIW, I've been running my i5-2500k at 4GHz since April, 2012. On air, it only hits 55C, regardless of playing AH2 or working in Photoshop or Lightroom.
So, 3.8GHz to 4GHz shouldn't be difficult for you, and relatively safe as far as OCing goes.