The Steam client by Valve software is now in the Kwheezy repository.
The Steam for Linux client gives you access to games in the Steam Store.
To install, open Apper and apply updates. Then type "steam" in the search field. The package is called "steam-debian".
The steam-debian package is courtesy of Keith Henderson AKA GhostSquad57. It was he and possibly the work of others that ported the steam client from Ubuntu to Debian Wheezy.
I've tested it with Nvidia proprietary drivers that come with Kwheezy (the Debain Wheezy version) and it seems to work great so far. Even on a recent graphics card (GeForce GTX 660).
If you do want to squeeze more out of your GPU, the latest nvidia drivers can be installed via smxi. Just switch to tty1 (Ctrl-Alt-F1), login as root, enter "smxi" and follow prompts to get to graphics (display/video) card driver installation. However, once you do this, you're no longer on pure Debian Wheezy, so do it at your own risk.
32/64bit architectures: Although I've only tested it on 64bit so far, it does support 32bit as well.