I've been manipulating imagery in 21601 × 10801 resolution recently. My record. This is using PS CS4 on my 2-year-old base MacBook(2.1 GHz Core 2 Duo, 2 GB ram on Mac 10.6.3.
A little slow generally. Flexify takes a couple minutes. Not too bad. Saving onto my 1.5 TB external drive is a bit excruciating, but that's 'cause the drive is cheap and slow. With my old 1 GHz single core PowerBook and CS1 8192 x 4096 was more of a challenge than this(actually painful in extremis dei, amen). I like my little bargain basement mac laptop.