According to steam I got this for the system requirements
Minimum:
OS: Windows XP SP3, Vista SP1, Windows 7
Processor: Core 2 Duo 2.5 GHz or equivalent
Memory: 1.5 GB
Graphics: ATI Radeon HD 3870 or NVIDIA 9800 GT
DirectX
Can someone explain to me the number of party members you play with at any one time. Some of the previews say you can only take one character and yet there is 4 player co-op. It seems a bit weird to have 4 characters allowed in co-op but only 2 in single player.