You can get by on 2GBs...but you'll find yourself running out of ram pretty quick - which get's frustrating.
If you want to run more than four or five apps at once, or if you want to run two powerful apps at once you'll be wanting 4GBs.
On my old 13" MBP with 2GBs, whenever I'd try to run VMWare + anything else my machine came to a standstill - lack of processing power wasn't the issue, it was lack of ram.