You can do 4 GB of RAM in the last gen. That's what I've got here. That said, a huge percentage of my friends that have had last gen models (including myself) have had a number of problems with the hardware, which wasn't true with the older PPC models.

The new ones are still too new to know if they'll be similarly problematic. I was disappointed with the new model not supporting firewire or more than 4 GB of RAM though, so I probably won't upgrade any time soon.