storing bitcoin's database directory on an external drive? I have a new-ish powerful laptop with SSD, limited to 128GB.
It came with a RAID which somehow didn't work for installing linux, so I had to disable the RAID and now I have actually two 64GB disks really.

That's just not enough to run the bitcoin client on it, the arch linux wiki recommends 80GB of disk space, and even ignoring that rather generous amount, it would fill up my disk.

Is it possible to run the client with a local wallet and all that but having the bitcoin DB on an external drive?
Bitcoind requires your wallet file to be in the same location as the rest of the files, so I think what you want to do will be difficult, as it might require editing the source/recompiling

However, you can just store your wallet on the external drive too. 

You can encrypt the wallet if you don't want it to fall in the wrong hands

You can use the backupwallet command to back it up if you are affraid of the quality of your external drive...