You dont neeeeed a new psu, you would be pushing it though as I said.
To explain the situation a bit:
1. Your 650w psu is a very good one, it can deliver up to around 800w before it shuts down
2. Most people neglect that that, in reviews, power usage is usually measured with a voltage meter at the power socket -->the actual power needed is that value x efficiency
3. Your system power consumption on prime 95 load without the gpus should be around 250w
4. A highly overclocked 7950/280 might draw up to 200w or so
5. You would about max out the rating of your psu on a synthetic, however you should consider that:
1. The power consumption for components can peak over 20% of the average consumption for a fraction of a second, thus a bit of a margin should be added in general
2. The voltage regulation at extreme loads suffers, which could lead to crashes or overclocking problems in extreme cases
3. Your PSU has a peak/overload voltage of over 20% its rated wattage and superb voltage control
4. Your psu will age a tiny bit over the years, if its at high load all the time it will age more obviously
5. A gaming load isnt anything like a full synthetic load on all components. PLaying Crysis 3 your pc would need maybe 450w.
Bottom line: I wouldnt say get a 650w psu if that was a new build but itll work.
That leaves the issue of crossfire support in GTA 5.....anybody got a crystal ball somewhere?