Ripping 700 CD's to HD using FLAC-How much space?

700 CDs will take up less than 300Gb. That's about what I've got.
I'm using two 500Gb external HDs right now -- one for the music, one for backup purposes. As you said, drives are cheap these days, so I may purchase another one as well.
I'm considering a NAS as well, but that may be overkill for my needs.

I recommend setting up a RAID mirror if you're going to rip 700 CDs. You might feel like it will never happen, but every hard drive was made to eventually fail. A NAS unit like the Netgear ReadyNAS will give you RAID on top of networking benefits. Or you could just buy a plain vanilla RAID controller for cheap.

Another thought here is, FLAC allows you to select the bit rate... so the overall file size will change accordingly.

I've heard no diffs from one bit rate to another in the 500 - 1000 region.

As it is a lossless codec it stands to reason you could even use the lowest bit rate encoding and save yet more space. The usual default of 768 or so seems appropriate enough though... and digital geography is mighty reasonable lately....

Do really consider having a back up however... Once you finish ripping all those CDs, and see how much time it takes to rip error free, and ever how many drives you will need to replace, you'll understand it better.

I rippped onto a 500GB USB drive... then got an Iomega 1TB NAS. The USB drive is now the back up... it's seldom if ever turned on save to archive to or from.

Have fun... and do them with error checking!