If you'd asked me a year ago I'd have probably said 2 GB unified memory would be best for next generation but already in 2008 you can buy 4GB for 80$ or so if you can shop around you can get it lower.
By 2011-2012 when I estimate we'll be kicking into next gen 4 GB will be significantly cheaper as PC's seem to get stronger every few months, prices reduce better things come out etc. I'd say in the next 3-4 years 4GB of ram will be cheap as I recall 2GB being 80-90$ not long ago which now I can buy 2 GB for... 20$ I'd say 2-6 GB is within reason pending how price drops go and the price of the system.
Edit: One of the best things they could do is not so much increase the memory as lower the latency and improve how it communicates with the CPU. Combined GPU/CPUs are in the future so that could be a cost cutting measure. I've always been a fan of unified memory though, it seems a better system as you can scale it as you need it, non unified if you're not using it for graphics, it just doesn't get used.
