EDIT: added Gamersgate deal.
Steam's daily deal sees the complete X-COM bundle reduced to £3.05, while the complete pack is £2.49 at Gamersgate. It's as if they think the whole internet is talking about the series. You probably don't want Enforcer and Interceptor but if you don't have them, you almost definitely want the other three. They are £1.01 each on Steam. I still play the original on a regular basis and find it laughable that it's available for a pound and a penny. HA!
There is no better way to spend a pound than on the first X-COM, that's cold, hard science. The great dilemma is not whether to buy, it is what to call the damn things. The original has always been UFO: Enemy Unknown to me, but Steam has it as X-COM: UFO Defense. But then is the hyphen necessary and is everything capitalised like that? Yes? Maybe. It's still Enemy Unknown to me.
Perhaps defense could become defence? And is 'USO: Fears Unspecified' a possible alternate title for the second game? No matter. Let's all play them forever and never win. While we're here, you could do worse than read Alec's impassioned love letter to the original game.