Bert has offered some excellent suggestions. Forget 32 bit and just get 64 bit ha. On SBS Standard, I usually say ~16GB for 10 users. For 20 users , 20-24GB is a good idea, especially when it's so cheap.
SBS Essentials needs a lot less since it doesn't use Exchange or SharePoint. Exchange loves RAM. You'll probably find when you start using SBS, that Exchange consumes as much free memory as it can find.
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