OK, in a nutshell without getting into all the particulars, the 630 with the "right" cooling and a little extra Vcore will do 4.5Ghz. Yes, the mobo is a big factor as well as the RAM. As you've probably noticed, my sig shows my 630 @ 4.2Ghz. Thats with a 1.41 (setting in BIOS) Vcore and water cooling. It's also Prime95/Super PI concurrent stable.
My idle temps with an ambient temp of 20c is 28c and @ 100% load it only reaches 42-43c. I could go higher with this CPU but my RAM is only DDR2 533 and already @ 560. My FSB is 280 and If your going to keep a 1:1 ratio between FSB😀DR (which I recommend for stability) your going to need some really good DDR RAM. PC4000 will get you to 250FSB if your BIOS will let you raise your Vcore above 1.4 but you need even higher performance DDR to achieve 4Ghz and beyond= big $$ for DDR which will not be forward compatible for high performance applications where DDR2 should be used.
Last but not least is your choice of cooling and I seriously recommend a nice H2O solution as all the people I've turned on to it have never had any regrets or accidents if done properly. The performance achieved is astounding and once I replace my RAM with DDR2 667 or 800, 4.5Ghz or better will be within my grasp. Eventually i will upgrade to a PD 930 and look to achieve similar results w/ dual core. If H2O is out of the question, buy the very best Air cooled solution you can find. It will be worth it.
The performance you will gain @ 4Ghz and beyond is incredible and I honestly would put my rig up against ANY single core CPU H2o cooled rig once I've hit 4.5Ghz or beyond. I highly recommend you OC to the max as you WILL NOT be disappointed with the performance increase. Good luck!