Only Photoshop CS5 and CS6 will install and run in 64 bit mode on Mac.
CS2 will only ever use up to 3-and-a-bit GB of RAM, even if more is available.
Only CS6 is available for purchase from Adobe and you can only upgrade to CS6 from CS5. CS3 and CS4 owners have 5 weeks left to upgrade to CS6.
So from CS2, unless you can find an earlier legal version with a valid serial number elsewhere, your options are: