I'm no engineer but I'm gonna try my best. The results are highly speculative, I know, but when has that stopped anyone here from dreaming ...
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Here SpaceX says that the Falcon 9 1st stage total thrust is 4.94 MN (vacuum), and each Merlin 1C engine has an Isp of 304 s (2.98 km/s, vacuum also). That means that that stage burns about 1658 kg of propellants per second. If the ignition time is 172s (from the 2nd launch video) that totals 281 t of fuel (all tonnes metric).
From the same page: Falcon 9 total weight at lift-off is 333.4 t, so the 1st stage dry weight, plus 2nd stage total weight, plus cargo, must be around 52 t.
The 1st stage fuel tank weights 9.000 lb (4.08 t, from this video).
Total thrust at lift-off is 5 MN and T/W ratio for the 9 Merlin 1C is 96, so engines mass has to be 5.3 t. That means 9.4 tonnes for the whole 1st stage (dry).
How much fuel has to remain in it to let it take down after separation?
At that moment the Falcon 9 is doing 3.2 km/s (also from the video), and if they want the stage to come back all the way to the launching pad they are going to need to counter the horizontal portion of that (I think about 2 km/s or so) and then give it a bit more.
SpaceX is planning to load its Grasshopper RLV with 26.8 t of fuel. From that much the present Falcon 9 1st stage, with Merlin 1C engines, can give a delta-v of 4 km/s at vacuum, or 3.6 km/s at sea level.
That's enough to decelerate the 1st stage. For the going down I'd use parachutes (they are already in their design) and leave just a bit of fuel for doing a powered descent for the very last part of the fall.
So, the question that remains is: will the new Falcon block 2 be able of reaching 3.2 km/s fully loaded, and still retain 27 t of fuel?
In their last August presentation SpaceX said that the Falcon 9 block 2 is going to be lighter (dry), using Merlin 1D engines with a T/W ratio of 160 (instead of 96 for Merlin 1C) and an Isp of 310 s at vacuum (vs 304 s for its older brother), and it will also carry more fuel.
I'll give it a little more thinking.