Aye a PA120.3 is enough for a 920 and 5870 on it's own, but I wouldn't bother with the extra hassle until your ready to re-do the whole loop as 1.
Remember radiators are the least restrictive components of a watercooling loop (bar reservoirs), so pump power isn't so important when it comes to adding radiator area... it's more a subject of diminishing returns.