AMD currently does not support DDR2. The next generation of Athlons will, but don't expect to see them until the middle of next year. While DDR2 will show promise, when AMD originally launched the Athlon 64, DDR2 was just starting to get out and was pricey, not to mention slower due to higher latency than DDR1.
DDR2's advantage over DDR1 is that it can scale to much higher frequencies, but to get this perfected and widespread has taken some time. At equivalent speeds, DDR1 tends to have the advantage because of the latency issue of DDR2, but once you get into higher speeds, like DDR800 and DDR1066, you start to see it's advantages.
We look at what offers the best performance, and especially for the money, AMD pretty much rules. Unless you were designing a system specifically for video editing only, Intel simply does not have the better performing CPU's at this point in time.
Unless you absolutely must have DDR2, that's something that I wouldn't worry about.
As for the 4600+ vs. the 4000+, that really depends on how much money you want to spend. Depending on your needs, yes, the 4600+ may be a better option, like if you heavily multitask with lots of CPU intensive programs. However, if you're just gaming, remember that the CPU isn't the only thing in the equation.
