Advice please - formatting a Mac Pro ... which operating system?

Hi,

My friend has given me a Mac Pro and I want to use it as a media centre in my living room. Basically for playing movies, music and surfing the web.

So I want to format it and install the optimum version, of an optimum operating system.

I am aware that MacPro's can support a number of different operating systems such as Mac OSX, Windows or Linux.

I'm afraid I don't know much about macs, I'm a PC guy myself. But I really want to learn more about them.

Can you recommend an operating system that would be best for my needs?

Literally, all I want is to watch high res videos(hopefully1080p), play music and surf the web.

The specs of my mac pro are as follows:

Model Mane: Mac Pro
Model Identifier: Mac Pro 1.1
Processor Name: Dual Core Intel Xeon
Processor Speed 2Ghz
Memeory: 5 GB
Boot Rom Version MP11.005C.B08


Any help would be very much appreciated.
Many Thanks

Any version will handle those tasks but you may as well go with the most recent (10.7 or "Lion").