Re: Burning smell...
The ram upgrade won't make much of a difference, If you want to be prepared for a next-gen (or high end current) card, I would recommend a 650-750 watt psu (with some decent amperage) (as newer cards draw 500 watts). However if a 550 works for you now, chances are it will last you another couple years in terms of power output with a similar build (Unless you are planning a larger card).
It may not be the ambient temperature of the PSU leading to the burning smell, rather it could be a single faulty component causing the board or shielding to melt. Is it like a heat burning smell or a melting plastic smell, meaning the overall air temperature coming out could be cool despite it burning.
Since you narrowed it down to the PSU, there is no need to run the tests I listed. (as I am sure you know, just letting you know anyway)