Both of your links point to the same product.
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.