At the moment I'd suggest you consider one or more of X-com, Dayz, Skyrim, Witcher 1 & 2, Far Cry 3 + Blood dragon rather than a pre-order you cant use now..

The thinking behind this is that they're tried & tested & well worth the money (Yes Dayz is in development - but there's enough content & game to warrant the purchase) Pre-orders are a bit of a gamble, hype can often be misleading & personally I'd suggest to wait for a number of reviews rather than gambling buying a pre-order - Yes Early Access is a form of pre-order, yet I recommend Dayz as an exception based on I've played the mod & SA extensively (plus its one of Steams biggest sellers)

Other steam games to consider

Running with Rifles

Mass Effect series

World in Conflict

Borderlands

Fallout 3 / Vegas

dayz

Total Annihilation: Kingdoms?

OMFG No one lives for ever, forgot about those, amazing games

10. Golden Eye (N64)

Unbeatable MP action, Great SP, years ahead of its time

9.Chaos ( ZX spectrum)

8 player wizard DM, enough said (currently being remade!!!!)

8. Freespace 1 & 2 (PC)

Two of the BEST space combat games EVER

7. Tie Fighter (PC)

Amazing Star wars game, probably the last good one.

6. UFO: Enemy Unknown (Amiga)

Original & best version, solid & playable even today

5.Doom 1 & 2 (PC)

The reason you bought a PC, fantastic for its time

4.Super Mario Galaxy 1 & 2 (Wii)

Flawless execution, such a great game(s) you have to play these

3. Civilization series (PC)

Best turn based strategy series ever, bar none

2.Half life 1 & 2 (PC)

Hlaf life 1 is the best SP FPS I've ever played

1. Elite (BBC Micro)

Outstanding game, looking forward to the new one

Honourable mentions:

Sensible Soccer (Amiga)

Deus Ex (PC) (original)

Fallout Series (PC)

GTA Series (Various)

Zelda Series (Nintendo)

Mafia (PC)

Super Mario (NES)

I agree with the Dead Space series, Outlast & Amnesia; others to consider that I don't think have been mentioned yet

DayZ

Clive Barkers Undying (quite old but REAL scary lol)

Doom 3

If I recall correctly PCgamer did a top 100 hooror game list around xmas, might be worth a look.

With FPS & single player, I'd suggest the following:

Far Cry (1st one as far cry 2 is ok, but 1st was muchbetter in my view)

The Half Life Series, even though the 1st is quite dated its still an amazing game. I would suggest also looking at the blackmesasource website (Half life remake)

As mentioned prior, Crysis & Fear are both worth getting. However personally i felt the FEAR series died at no.2, Dead Space is really good, ret it.

Return to Castle Wolfenstien (if you like quake/doom you'll like this), also Prey was damn good.

Mass Effect 1 & 2: not strictly FPS but amazing & both are a must play!

Mafia: one of the best story driven games ever

Brothers An Arms series (latest one is really good)

metal fatigue?

http://en.wikipedia.org/wiki/Metal_Fatigue