Memory makes virtually no difference. DDR3 is fast enough so about only reason to use DDR4 is lower prices, bigger modules, lower power consumption and better support for future. Speed wise there is no problem still using DDR3. i7-6700 would make computer faster but it's still quad core vs quad core so not That much faster especially when you are using software that utilize all cores.
i7-6900K is way overpriced. We don't know yet how expensive Ryzen will be and also there are likely 6-core version coming also. For pricing, we must consider following:
- Ryzen motherboards are much cheaper than LGA2011-3 ones and about on same range as LGA1151 motherboards.
- Ryzen die size is smaller than die on Radeon RX 480 so Ryzen is cheaper to produce. Not to mention with Ryzen there's no need for 8GB memory, VRM components etc like on RX 480.
- There are big bunch of morons that buy Intel even if AMD is faster, cheaper, cooler etc, we saw that on Athlon64 era.
So Zen is expected to be quite cheap. I wouldn't be surprised if AMD sells cheapest version of 8-core Ryzen for $350.
Anyway, since there's only three weeks for Ryzen official launch, it pays to wait for it.