Short Answer

Minimum System Requirements:

• 1.0 GHz x86 or x86_64 Processor
• 192 MB of system memory (RAM)
• 2 GB of disk space (Frontend Role)
• 20 GB of disk space (Backend Role)
• Graphics card capable of 1024x768 resolution
• Supported TV Tuner Card (Backend Role)

Recommended System Requirements:

• 2.0 Ghz x86 or x86_64 Processor
• 2GB MB of system memory (RAM)
• 10 GB disk space (Frontend Role)
• 160 GB+ disk space (Backend Role)
• nVidia 8xxx+ 256MB+ Graphics Card
• Supported TV Tuner Card (Backend Role)

Long Answer

Long answer: MythTV hardware page

Still not sure? Take a look at the hardware the Mythbuntu developers use.