I've played both Mass Effect 1 & 2 on Xbox I but never made it to #3. It is a good game and even better that it's free.
Im not sure when this expires but I would say that they wont give it away for too long.
You can buy the trilogy for $29.99 which also includes Pinnacle Station and Bring Down the Sky but try #2 for free first.
I have not played Dungeon keeper but they are giving it away (was $4.99) so worth a download.
Mass Effect 2 you will need the following minimum system requirements:
OS : Windows XP SP3/Vista SP1, Win 7
Processor : 1.8GHz Intel Core 2 Duo [equivalent AMD CPU]
Memory : 1 GB RAM for XP / 2 GB RAM for Vista/Win 7
Hard Drive : 15 GB
DVD-ROM : 1x SPEED
Video Card : 256MB* [with Pixel Shader 3.0 support]
Sound Card : DirectX 9.0c compatible
DirectX : DirectX 9.0c August 2008 (included)
Input : Keyboard, Mouse
*Supported Chipsets : NVIDIA GeForce 6800 or greater; ATI Radeon X1600 Pro or greater. Please note that NVIDIA GeForce 7300, 8100, 8200, 8300, 8400, and 9300; ATI Radeon HD3200, and HD4350 are below minimum system requirements. Updates to your video and sound card drivers may be required.
Dungeon Keeper you will need the following minimum system requirements:
• Windows XP or Windows Vista
• 1.8 GHz Processor
• 512MB RAM (1 GB recommended)
• 3D graphics card compatible with DirectX 7 (compatible with DirectX 9 recommended)
• 2GB HDD