Hardware suggestion to increase workload for current VM

Hi,

I am using ESXi 5 on my current config as:

CPU: Q9550 / MB: Asus P5Q-EM / RAM: 4GB DDR2 / HD: 1TB x 2 (Raid1)

I have 2 x WinXP for user to use RDP running small application (concurrent users < 5) + 1 Win Serv08 R2 for Anti-virus server.

I would like to add 1 more Server 08 R2 server on it for another application server (running 1 application with SQL express, concurrent connected user <5).

1) Please suggest if I increase totally 8GB ram to the server can support it?

2) As it's difficult to find a MB for Q9550 that support DDR3 ram, I still need to upgrade the DDR2 Ram, it is worth?

3) Is it better to move the VM storage to my NAS (Buffalo Tera Station with 1TB x 4)? Or using the current HD inside my VM server? (Storage size inside VM should not be too large)

Thanks,

Jerald