I'm often running several VMs at the same time on an I7-2600 with 16GB RAM and from my experience, it's always the memory which gets exhausted, so go with the 32GB if you can afford it. I'm also using two SSD's (Samsung 830 connected to a 6G SATA interface), one for OS and personal/dynamic data and one exclusively for VMs and a virtual iSCSI target. All other data as well as some rarely used VMs are on a 2TB HDD. IMO there's no need for RAID, at least no RAID-0! I'd rather use a second HDD for backup purposes.
To answer your questions:
1.) RAM settings for what? For VMs it depends on the guest OS.
2.) on the system disk/SSD
I don't see the need for using a RAM disk with SSDs in place.
André