Cheers for the responses, I think that i'm going for the 64's just on the factor of money and i found that (according to a review) that the 4800 64 is just a little quicker than the 175 (2.2Ghz) Opteron.

I would love a boxx (In fact I was thinking of the Boxx 1400 series, but found that its exactly the same as the AJP 900 64 series but quite a few pounds more expensive) but alas there out of my price range.