Correction: Only Fallout 1 is entirely free. Fallout 2 requires a pesky subscription.
If you want to go into further details about the series, there's also a Fallout Wiki
The only thing that is tempting me to buy F3 is, well, mods. There's quite a few out already, and many of them try to inject the original Fallout "feel" into F3. However, the only thing stopping from getting F3 is that it's been in the hands of Bethesda, so I'm expecting cheese galore with the voice acting.