the blockchain right now is not 600mb, its more like 400, excluding the index files. and that can be compressed to at least 80% of the original size.
and. 16 gigs should be good for a linux install and 2 more years worth of blockchain worst case.
It is my understanding that the nodes save multiple copies of the block-chain in case of a split or one of the block-chains becomes the "longest" one. I have had a test-node running since June 9, 2011 (0.2.22 and 0.2.23) for a total of 55 days. It ran out of disk space today; consuming 5.8 GB. That works out to 105MB per day. Disk usage dropped to 4.9GB when the client exited. The client had 125 connections during peak times.
No, it doesn't save multiple copies. When there is a fork, it keeps both blocks, but it doesn't need to make a copy of the rest of the chain to do it.
Check to see if you have a debug.log. If I don't clear mine often it gets huge. Currently around 1.5 GB.