The thing is you're a bit late to buy DDR2 which has nearly doubled in price. DDR3 is priced very similarly. So it makes sense to get new tech DDR3 if you're building a new rig. And yes, 4GB DDR2 is about $100 US.
It depends on your target o/c. If it's 3Ghz, that'd be 9 x 333.33 = ~ 3Ghz. That means 333 x 2 = 666Mhz ram will do. You can get the 1066mhz ram but your heat will be the limit. It's 65nm.
Either of the above suggestions is a better buy. I'm leaning toward the HK for its low clearance. The other tall sticks MAY interfere with some big cpu coolers, like those covering the ram slots as well as the cpu.