If you have a ps2, the 40 gig is your best bet.
If you must have ps2 backwards compatability, wait for the MGS4 bundle with the 80 gig that has software backwards compatability.
Stay away from the 20 gig ps3, especially if you do not want to bother with upgrading hard drives. Games coming out are going to be requiring 4 GB installs, and I'm sure later on things will even get worse. You will be hurting with space with the 20 gig version.