Thanks guys!
Simon, yes I've looked at the C6 but it's lacking some features such as powered x,y carriage and is less powerful. Also the 320 I'm looking at has 38mm spindle bore which is significantly more than any Seig machine.
Mark, you make some good points about the C4 and it's a very tempting machine. I didn't know that the 320 I'm looking at used to be sold as school machines or that they are out of the same factory, that sounds like they'd be reliable. I think the main reason I'm keen on the 320 is that it has significantly more capacity, and I don't want to have to upgrade later. I just hope this extra capacity doesn't come at the cost of accuracy.
I have looked at the C8 too as carbatech say they can get it in for around $1600, but it still doesn't seem as good as the 320.
Thanks guys, feel free to give more advice, I'm a complete noob at all this!
David.