I'm not sure how many Gigs and CPU cores I need to have in my system. Any recommendation?
With using more CPU's with Cores and Cache only improves the IOPS in NAS for read and writes also for many users connected to the NAS head. If there are 250+ users 1 Quad or 2 will perform better. This is a difficult matrix to place as every environment is different but in general for low amount of storage of 4 - 8TB then 4GB is ok with 1 Dual Core (fast w/ high cache).
Generally speaking, more processors (as opposed to processor power) will perform better when there are many simultaneous connections to the server at once. More memory can be used with NAS and file I/O iSCSI (for caching purposes).
Your choice of RAID is also dictated by your requirements: if you value speed over storage space, you should probably select RAID10, otherwise RAID5 or 6 (depending on your redundancy needs) would be suitable. The assistance of the RAID set like RAID 10 improves the performance as well as the cache that is properly set for high IO.
We support up to 64GB of memory for 32bit version. However, if you are using 64bit version ram amount is unlimited. We use all available free memory strictly assigned to cache, the cache will rarely be released during write or read options until you reboot the system. The reason is to increase operation performance for the resources you are using when utilizing features of Antivirus, Backup, Data Replication, Snapshot and Volume Replication, so the outcome is preferred when the cache is available all the time instead of drawing from it when needed.
Though depending on the amount of functions you plan on using we see many customers perform very well with 4GB of memory with even up to 12TB usable storage. Though with today's demands I will limit this to 8TB.
Also another good reference to a memory recommendation is our Knowledge Base article below.