I've decided to go for the HD5770 in Crossfire as it seems the best bang for buck atm and will last me the 2-3 years I want it to, not that I will have this PC for that long.
My 8800GTX lasted me almost 3 years, and to be honest would still be fine for most games at 1680x1050.
As for a dedicated physX card, if there ever came a day that I changed to nVidia cards and would need/want that I would get me a 9600GSO for £45 to fill that role.
Saying that, nVidia's GTX300 range will probably be more than capable anyway when it comes down to it.
Thanks for the advice people. Much appreciated.