Hey look, another Half-Life 2 mod, because we don't have enough of those already.  Actually, no, we really don't.  But Bobfish, why? I hear you ask.  And the answer is simple – because it's Half-Life.  Anyway, this latest iteration of the Cinematic Mod, version 12.1 boasts some jaw dropping visuals, and the ability to not only fully customise which character models you do and don't (ahem, Asian hooker Alyx.  Why, just why?) use, but also offers alternate costumes, and the option to have characters change them at various points throughout the game.  And that is truly just the tip of the iceberg.  On top of the eight(?) gigs or so to install the game itself, the mod takes up a whopping thirty! That's a monster of an update, and then some.  Not to mention the increased system draw.  On top of those listed below, I've done a little digging to find that people are recommending a hefty quad core on top of that.

Hardware requirements:
DX9 capable graphics hardware with minimum 512 MB VRAM. (1 GB recommended)
3 GB RAM (4 GB recommended)
Software requirements:
Fully installed Steam client. You can't start the mod without Steam running.
Fully installed and unlocked HL2 / EP1 / EP2. Make sure, you have started those games in the past.
Source SDK (SDK, Base, Base 2007) installed (you can get them in Steam | Tools).