Based on my experience there is no golden rule neither for CPU nor memory...
Its all based on the usage...if you have 1000 active users then you need maybe 8G or RAM and dual quad CPU. If you have 100 active users, snapshot, data and volume replication then you also needs more CPU and so on...
Generally, 3-4G RAM and dual core CPU, would be good enough for average usage..
well, its clear for me. that hardware requirements are depend on a load, but my question is also about the problem that generally using RAM beyond the 3GB level on 32-bit systems is a little bit tricky ...