Efficiency is irrelevant. 14 amps output is 14 amps output, there will just be more input power draw off the wall with less efficiency. You don't want to be much above 80% of the max power or overwork the psu. A good quality 250w may be fine (I'd still suggest at least a 300w in this case) but this is just an ok psu.
I too purchased the Acer Aspire x1420. I was following your conversation and you guys seem to have agreed that the HD7750 low profile is a little much. What would be the best graphics card budget-wise and power wise I can safely use on this computer then? Thanks for your help.