GameTap has Fallout 1 free (for now) & Fallout 2 for subscribers & Tactics scheduled to be added soon
or
GOG.com (like RaiderJ wrote above)
or
gogamer 48 hour madness sale
Fallout Collection $12.90 + $2.99 shipping
gogamer link
edit: I ran FO1 and Tactics this Spring on a bottom-of-the-line Dell outlet special with Vista Home 32bit and integrated graphics without any system problems that I remember. Save often in Tactics, though, as the game itself has problems regardless of the OS.
Agreed with RaiderJ (above), that FO2 is the best of the set. I played FO2 first, way back in the day. Played FO1 years later, when I gogamer'ed a FO1/2 Dual Jewel Pack for $3+shipping in Jan2005. Also bought Tactics years ago, but never completed it until this Spring.