Correct. The P5W DH Deluxe is FSB limited with a Dual Core processor to ~ 425 Mhz at best, on a good day, if all the planets are aligned, so 4 modules will further limit the memory controller on the 975X northbridge chipset due to the additional current required to communicate with 4 slots. The Q6700 is the best Quad for that board, due to the 10x multiplier, which allows the FSB to run at only 360 Mhz with a very nice overclock of 3.6 Ghz. On the other hand, the Q6600 will never reach 3.6 Ghz due to the 9x multiplier, which requires 400 Mhz FSB.
Also, northbridge temperatures can be reduced and FSB overclockability and stability can be enhanced by removing the very shiny, but airflow-limiting cover from the northbridge heat sinc, and attaching a 40mm fan - http://www.newegg.com/Product/Product.aspx?Item=N82E168... - to the heat sinc. Simply usee the fan and screws from the kit, which will attach quite nicely to the notrhbridge heat sinc.
DDR2-533 in dual channel gives a Q6700 FULL memory bandwidth and doesnt require any higher spec memory although for overclocking and what not it wouldnt harm to get higher spec'd ram and DDR2 is so cheap anyhow
your best bet is 2x2gb 800mhz DDR2 to get a total of 4gb dual channel etc
if your overclocking a Q6xxx you will see a limit of around ~3.4ghz on air depending on sample, and you can simply raise the FSB from 266 (1066) to 333 (1333) to get 3.33ghz and should be fine on stock cooling, or even if you lower the multiplier from 10 to 9 to get 3ghz, and stick below 1.45v if anything (shouldnt need any extra v's anyhow)
as for 45nm cpus on older motherboards - there not designed to run on there so yeah no guarantees etc, you best bet for a simple upgrade is an overclocked quad