Tactics is a spin-off game... Its a mission based combat game that greatly improves the basic TB combat mechanics in Fallout, but turns it off by default
It fails badly as an RPG ~But then I don't think its trying or claiming to be one. Unlike Fallout, you basically just do the assigned mission and return for another one.
It cannot compare to the other two in their own element, but combat-wise, its marginally superior.
The CD's are available in online outlets, and any PC made in the last 12 years should be capable of running the first two if not Tactics.
However I'd still recommend the GOG versions... They work without a hitch... Fallout 1 & 2 cd-versions might not.