16amps on the 12v is pretty low.
if u have a couple of case fans, and 2-3 HDD, then i think a 6800 is pushing it, probably out of the question. i think kogan is right with 6600 class (or lower) of card.
on the Ati side, you may be more lucky. ATi have a staggering number of cards to chose from , X800pro, X800XL, X800GTO, X800GT lol.......loads. alot of the mid-low range Ati cards are made using 11nm process. i reckon a something like a X800GT/GTO would work, and i think anything from the last gen would work. (my 9500pro works on a 400watt psu with a paltry 12 amps on the 12v) maybes even a x800xl? (its 11nm) you'd have dig up reviews that include power consumption.
but if you havent bought this yet....then try aim for something a little stronger. again like Kogan has said you want to be looking at Antec, enermax, Fortron, for good quality PSU's that dont dent the wallet
but basically your looking at the Mid range
6600's and lower for NV (goes down to 6200's which for the money arent bad imo)
X800XL - (you may get away with but dont hold me to it)
X800GT/GTO - basically same ball park of performance as the 6600's
theres X700, but in all honesty its not really up to snuff, and you'd be better off with the ones above
X600 is a rebadged 9600 from the last gen, probably best to aboid if you want some kind of life span