Honestly, the reasons I went 8gigs, is because:
1. If I decided to use nas4free, I really wanted to use ZFS (which is RAM hungry)
2. I figure in the year 2147 when btrfs is considered stable,
, my machine would be ready to use it. Again, btrfs, like ZFS, is pretty RAM hungry (or at least it's presumed it will be..lol).
3. The RAM was super cheap on sale at Fry's. I could either spend $33 on 4gigs, or $40(just realized I typo'd the price in my first post, it was $40, not $49) on 8gigs (or dipped down to 2gigs for around $23). This seemed like a no-brainer to me.. especially if I ended up wanting to use ZFS or btrfs.
However, in light of the fact I'm not using ZFS or btrfs, yes, 8gigs is way more than I need. I could probably get by pretty well with 2gigs minimum, and 4gigs I'm sure would be more than acceptable.
One more thing.. if you go with building your own NAS... If you decide you don't need hair anymore and want to go w/ FreeNas.. it can be installed to a thumb drive. Same with nas4free. If you decide to go with OpenMediavault or you'd like to support a product that hasn't been upgraded in 10yrs (OpenFiler), they cannot be installed to a thumb drive (unless you only want it to last about 2 weeks). They also require an "entire drive" to themselves... in other words, you put them on a drive, and that drive is only used for the operating system, not for file storage.
For this.. I had several 2.5in SATA drives here from broken laptops/netbooks, etc... so I just put OMV there. If you don't have one.. You can get new 40-80gig SATA drives on Ebay for $35-$45 bucks all day long. Those drives aren't good for much now days, but are perfect for hosting the OS.