For PC... I highly recommend to get the pc version for any first person shooter. I doubt they even play on the same network online because you have so much more control with a mouse that the x-box players would just get destroyed.
And it's purely an online game, there's no point in getting this if you're going to play single player or even without voice chat imo.

It's not very resource intensive. A friend of mine is playing it on a 5 year old pc without too much trouble. geforce 7800GT+ should be fine.