ZFS on backup fileserver - RAM usage

[Miroslav Lachman]

I am planning to install new server for backups with 4x 1TB SATA II 
drives in RAIDZ. There will be about 20 separated filesystems from one 
zpool, few jails with ssh (scp/sftp), rsync and maybe FTP daemons, no 
other services with huge RAM utilization. As FreeBSD 7.1(-BETA) amd64 
still have some limits of kernel space memory, are there any benefits to 
put more then 2GB or 3GB in this server? Will it be more stabel or 
faster with for example 6GB of RAM? (I can buy it, RAM is really cheap 
in these days, but will it have some sense or is it vaste?)

I am using this tuning on testing machine (with 2GB RAM):
vm.kmem_size="1024M"
vm.kmem_size_max="1024M"
vfs.zfs.prefetch_disable="1"
vfs.zfs.arc_min="16M"
vfs.zfs.arc_max="64M"
kern.maxvnodes="400000"

(recommendations from http://wiki.freebsd.org/ZFSTuningGuide)

Have somebody better results with another values?

Miroslav Lachman