I would hang fire on memory for the moment, there is plenty of time, just see how Stardock is going to put the extra memory to use first, and how they will do it. I suspect Win 7 8Gb will be the new "practical minimum" (they will not force people to buy more than 8Gb to keep up minimum performance levels). Above that will depend on what they ultimately do with it. I suspect 16Gb will be the maximum they will play with on mainstream, any more than that and it forces people to go Win 7 Pro (with its 192Gb max memory) and they will not do the latter as a compulsion.
There is little doubt they will make it worthwhile getting more than 16Gb, but not "mainstream" as it would force upgrades to Win 7 Pro, the latter's not happening as a mainstream proposition.
I suspect 8Gb will be the sensible minimum, with 16Gb nice to have, and 16Gb + a nice bonus. There is no rush to buy it, it will be a while before even the Beta "needs" the higher quantity of memory. Just "hang loose" for a while, and buy a bargain, but otherwise leave it until mid next year if you are currently on 4Gb. If on 8Gb now, leave it, only buy more if final performance in a year's time dictates it. Just bare in mind that more than 16gb will need Win 7 Pro .....
Also worth buying higher speed memory if shelling out for more anyway, it does have significant benefits over 1600ns memory, just make sure your maotherboard supports the higher speeds (2133 or 2400ns - higher speed than that is marginal and you pay through the nose for it). Do the homework on memory above 1600ns, not all boards support it, and you need to make sure the specific speed and memory manufacturer is supported by the motherboard.
A nice practical high end (ish) memory as a starter for ten: http://www.overclockers.com/gskill-ripjaws-ddr32133-8gb-ram-review/,
However, generic memory is fine for 95% of people, don't knee jerk high end matched memory unless the motherboard can really use it to the full.