Just so people know, I did get the card, but unfortunately when I put it in I figured out pretty quickly that it was 256MB.
The good news is that the guy I got it from is a pretty top bloke so he said I could just use it in my system for free until I had enough cash for an upgrade!!! He said it was his mistake as he did think it was 512.
It's a nice upgrade from my 7900, TF2 for example runs much smoother, but I can't help thinking that the GT is a bit crippled by the framebuffer, as even at 1440*900 I can't really use more than 4*AA in most of my games when the action heats up, which is what I had it on before.
Still for free, can't complain I guess