1. your motherboard supports up to 16GB, so you can add more memory. But check hw-compatibility list for supported modules.
2. that depends on how much you have to pay for ddr2-ram. I'd try to get 2nd hand (i.e. ebay).
3. depends on network speed/latency and NAS configuration. If your NAS is attached to esxi by 1gbit, I'd say it is about the same. But NAS gives you better protection against HW failure (I suppose you have those 4x 1TB in RAID)...