Archeon - the linux-raid mailing list is very helpful There is also the wiki I set up : http://linux-raid.osdl.org/ which is the semi-official linux-raid FAQ nowadays.

Linux raid5+6 can grow by adding disks or by replacing disks with larger ones.

Adding disks of the same size is trivial, install and 'grow' onto the new disk.

Upgrading all disks is fairly easy - just remove a disk and swap in a larger one, then resync the array. Do this for each disk. Then tell the array to 'grow' into the new space.

Upgrading your mb - not a problem.
Moving from SATA1 to SATA2 - not a problem.

Anything else?

Frankly your biggest requirement seems to be flexibility - Linux SW RAID has that in bucketloads!

(I just installed my 4 new 1Tb disks today; I also have an old 1Tb RAID6, a MythTV on a 1.2Tb RAID5, a 600Gb mirror and a mirrored / somewhere.)