Currently Being ModeratedJan 31, 2013 8:42 AM (in response to simonjw)
Best Value - Mac mini, Quad i7 with 16GB of RAM (don't buy from Apple, install it yourself).
If you need a monitor - 27'' iMac, BTO it depending on your personal needs.
Don't buy a Mac Pro. They're overpriced and underpowered. Wait till WWDC 2013 if you want to buy a Pro.