Just like freebsd/Linux/Unix there can be one or more page files in OSX and while the SSD makes this less costly performance wise than with a hard drive, it's still nowhere near the performance of RAM so you still need enough ram.
Casual testing I've done indicates that for my usage patterns, 2gb gives good performance but 3gb offers a significant gain, and 4gb gives a much smaller gain.
Lesson is don't scrimp on ram. Requirements and base system ram goes up pretty consistently over time and since MBAs are not ram upgradable I'd highly recommend the 4gb configurations.