1GB is 1 billion bytes worth of data I believe.... is that what you were asking? DDR3 is probably not the best right now due to high latencies. But it will be. Get two gigabytes ddr2-800 since it is cheap but still good, then upgrade once ddr3 is worthwhile.
Yes, a blu-ray drive is the next generation dvd drive, so it would take the place of a dvd drive. You really don't need it though.
OCZ, Corsair, Crucial, Kingston, maybe Patriot or G-Skill.
Yes, Ultra is the best. It's not really worth it though. It's about six hundred dollars compared to 250 or less for a GT. And the performance difference is not too much. Perhaps 15%.
Don't know about best HSF, probably a Zalman. Ask in the CPU section. However, a cpu comes with a HSF that works well. You don't need to buy one unless you are overclocking.
Fastest hard drive would be a western digital raptor. Or a raid array, which is basically two hard drives seen as one. The computer writes to both at once to double the write speed.