go with the Xig s1283 non dark knight version.. unless you plan to upgrade to the i7 later on down the road.. the extra cost for the DK version is for the fancy looking fan and the i7 mounting kit..
with the stock cooler.. your best bet is to see how high you can get it at stock voltages.. but you could always try to push it.. you wont kill the processor unless you try to exceed either 1.5 or 1.6v.. i cant remember exactly what the recommended max voltage is for the core 2 65nm processors.. but just go with 1.5v as the max until you find the exact number..
when overclocking.. make sure you watch your load temps.. and stress test using either prime95 or intel burn test.. intel burn test will be much quicker to use.. just run 20-30 passes and that will pretty much tell you if your overclock is stable..
also ignore the idle temps because they are wrong.. its a known problem with the core 2 processors that the idle temps are incorrect. also be warned that running with all 4 dimm slots full may limit your overclock.. you might have to raise the northbridge voltage to compensate for the extra load..
if you need any further help on overclocking post back your questions and either i or some one else will be able to answer them..
main/gaming/distributed computing rig
phenom II x4 940 @ 3.5ghz 1.476v, Biostar TForce 780g m2+,2x1GB corsair XMS2 and 2x512mb kingston generic ram 3GB total, evga 8800GT 1.1v mod (756/1890/1015), Antec 300 w/ 5 tri-cool fans on high. 3.2TB HD space
HTPC/distributed computing rig
AMD athlon 64 x2 6400+, Asus m2n-sli deluxe, 2x1GB Crucial Ballistix Tracer 2.4v, Powercolor 8800GT accelero S1 (630/1836/1050), Xclios Windtunnel version 1 case