The back of my PC is against (well, about a foot away) from the wall with the radiator on, but it's over to the right a bit so shouldn't really make any difference.
That's a good idea with the outside air thing mate, I thought exactly the same after seeing Sam's comment
You could even try and get a fairly big fan and put that in it somewhere or on one of the ends so that it sucks a bit more air through as well, obviously at low revs though seeing as it'll be a biggun plus it'll stay quiet
We used to have some of that flexible pipe stuff but I think we chucked it years ago
