The OnLive gaming service is out of beta and will go live June 17th for about $15 per month granting you access to the entire catalog, considering you have a broadband connection.
The gaming service should be accessible by both Mac & Windows and can "offload" the entire performance aspect of the game to 15 different remote server strategically located to reduce lag allowing most computers to run games at 720p.
As a bonus, the first 25,000 people to sign up will get their first three months free.