Q6600 instead if it saves you 50 quids or more. Q6700 is just a higher mulitplier. If you do air cooling, it will o/c to more or less of what a Q6600 can on air. If you have exotic cooling like LN2, DICE, phase, Q6700 or even QX.
P45 instead if it's about 20 quids extra. If you want the best P35 overclocker, DFI LanParty UT P35-T2R, 2nd to none. However, it's not for the faint of the heart. Look at GA-P45-DS3L/R or P5Q-E.
Actually, if you overclock Q6600/Q6700, any DDR2-800 will suffice. 9 x 400 = 3.6ghz is about what they chips can top out at on air. If you have exotic cooling, go with 1066mhz ram or faster.