I'm starting to get a little confused here. Yes there are motherboards out that support DDR3, and there are also motherboards that have support for both
DDR2 and DDR3 (not at the same time, though). However, the performance on DDR3 RAM has not yet given the yields that justify the high price and poor timings. A good set of high-speed, low-latency DDR2 RAM should be more than sufficient for this build. By the time DDR3 is worthy of use, it will most probably be another upgrade time for you (and me
Can I ask why you would ever need so much RAM? 4GB is plenty for advanced users right now, and a waste on anybody else. If you really want 8GB, then I suggest grabbing one 2x 2GB kit first, and if you find that you're using all of it and need the rest, then grab another 2x 2GB kit of the same make/model of RAM. All you would need for this is a motherboard with 4 RAM slots, support for up to 8GB of RAM (or more), and an x64 operating system.
I like the Seagate 7200.11 hard drives, I believe this to be a good choice.
Yes, Corsair is one of many quality manufacturer's of RAM. Other brands include Patriot, GeIL, Mushkin, G.Skill, Crucial, Kingston, OCZ, and a few others that don't come to mind right now. However, each manufacturer has a few different ranges within their product lines, so be sure to research exactly what your needs are, what capabilities your motherboard supports, and match these together.
Case fans are just that, case fans. They blow and help keep things cool inside. But of course, good in-case airflow is absolutely crucial
to the well-being of a desktop system. Even if you have the best fans in the world - if they're set up wrong, they could be causing air to "pocket" inside the case, potentially allowing a component to overheat when otherwise it would not.
Liquid cooling is very effective, but it must be done right. This is one of those "Go all out, or go home" scenarios. Unless you're prepared to spend hundreds of dollars on a high-end water cooling system, you could end up damaging your system (leaking pipes, poor quality materials etc...). A good heatsink/fan setup is fine for most systems.