I have a 2.4GHz MacBook Pro running 10.6.4 with 2 GB RAM which should be broadly comparable.
It's enough for PSCS5 if you're not using anything else at the same time. Same goes for Lightroom. Run both together and it can slow down appreciably as the memory runs into swap.
Photography isn't the main purpose for the machine (database stuff mostly) so it's not such a priority, but I am certainly thinking of going up to 4 GB. 64-bit apps do seem to grab noticeably more memory than when running in 32-bit mode (I'd estimate around 25%).
It may depend partly on the size of the files you're dealing with and how many you're likely to have open at the same time, but I reckon you could get away with 2 GB, but go for 4 for peace of mind if you have the funds.