ReadyNas Pro 6 - Unit Failure

The NAS drives are not in a format your PC can read directly, so your "pull it and read the data" plan is flawed in that regard.  You can always go with RAID6, with two redundant drives.  That still gives you more storage than 3 RAID1's and also keeps you from having to manaully balance the space between separate volumes.

Memory is not as piicky as some would make you think.  I've upgraded units with several brands of PC2-6400 SIMM (full size).  It's only expensive if you choose to go to 8GB, as 4TB 1x8 DDR2 memory is rare (because 32-bit PC's of the day couldn't use more than 4GB and had at least two memory slots).  I've bought GSKILL and OCZ CL4 from eBay and it worked fine.  Just don't get the OCZ with the tall heat sink.

Power supply is a standard SFX supply, but it does need three 4-pin Molex connectors and an extra long main 24-pin cable (usually obtained via a 6" extender).  The original is a 300W, but does have more 12CV available (for the drives) than many 300W.  I recommend you go with a 350 or 400W.  I typically move the 90° Molex from the original supply to the new one, which makes for a cleaner install as well as letting me have 3 by replacing SATA connections with them.  While the original Seasonic supply is no longer available, the SFX form factor was used in some small-profile PC's and is still readily available.  Some have a fan offset from the center, whihc others have claimed was no issue.  I've seen some that claim to be "SFX" but have the fan on the side with the AC power input and switch.  that's not true SFX and won't work right.

A quad processor is overkill and I know of nobody who did it and stuck with it.  They generate too much heat and, unless you are willing to cut a hole in the side of your NAS, the heat sink isn't upgradable..  If you have the older Pro Business Edition or Pro Pioneer, then you have a Pentium E2160 @ 1.80GHz.  That's pretty slow and upgrading to a Core2Duo E7500 @ 2.93GHz is pretty easy (assuming that's in your skill set since you mentioned it) and they are cheap on eBay.  It has the same TDP as the original.  The (slightly) newer Pro6 has a Pentium E5300 @ 2.60GHz, and IMHO you don't get enough of a boost going to the Core2 to bother.  You can check what processor you have in dmesg.log in the log .zip file you can download from the NAS.  Note that I believe the latest BIOS is required for the E7500.  You can go up to an E6600 with the older BIOS. .

RAID is not backup.  It provides continuous access through a drive failure, but anything taking out the NAS could also take out all the drives.  You could use cloud backup if restoration time is not a concern.  Of course, fire, flood, and theft aren't protected against with local backup, either.

You could use a USB chassis that has built-in RAID for your backup if you are worried about growing the backup.  Unfortunately, the Pro has only USB2, so it's pretty slow.  You can actually go faster via Ethernet to a backup on a USB3 equipped PC.  So if your storage and backup needs grow, you may want to get a faster NAS and relegate the Pro to backup duty.