I'm running 8Gb of ram on a 64bit sytem and I'm running out of memory for the large scale terrains I visualize.  Currently, a 12km by 13km area, populated with tree and vegeration eco systems tends to choke and blow up for me unless I'm very selective on my camera angles - in other works, don't peek over into the next valley and keep the clouds to a minimum.

I'm waiting for the news of the new Intel systems to come out - in a week or so.  The Intel X79 chipset will allow up to 64Gb of ram on the modo (with 8Gb sims - 32Gb w/ 4Gb sims).  I'm less enthused about Intels' pricing strategy.  Since AMD botched the Bulldozer CPU, Intel has decided to not sell the 4-core version of the new CPUs until the spring - which would have been the more cost-effective way to go.

For those of you running out of ram, the X79 chipset boards may be the way to go, but you may want to wait 6 months until the $350 CPU is released.  BTW - the upper end X79 mobos have 8 simm slots.