Help with power-loss protection using capacitor
Like ajs410 and Thomas say, using diode drops to go from 5V to 3.3V is a Bad Idea™. That's because, despite what you've been told in school, a diode voltage is anything but constant. 
The 3 diode drops may give you roughly anything between 2.3V and 3.2V, which may or may not be too low for your \$\mu\$C or SD-card.
I would start by replacing D4 with a Schottky type like a BAT54, which has a low leakage current of &lt;1\$\mu\$A typical. This will give us a few hundred mV extra for the buffer capacitor.

Next there's the 3.3V power supply. Use a low ground current LDO, like the Microchip MCP1703, which has a ground current of just 2\$\mu\$A. (The Seiko S-812C40 is a favorite of mine and has even better specs, but seems to have poor availability for low quantities.)  

Then you want to detect the loss of your 5V power supply. For this I usually use a MAX809. This will create a low output signal when its input voltage drops below a certain threshold. For a 5V supply threshold voltages of 4.63V, 4.55V and 4.38V are available. The output of the MAX809 goes to your \$\mu\$C's interrupt pin, so that you're immediately warned when the 5V drops, and you can write the buffer to the SD-card without delay.  

Now there's only 1 point left: the size of the buffer capacitor. You need to know how much current you're drawing from the 3.3V supply when writing to the SD-card. Let's assume this is 20mA. The capacitor voltage will decrease linearly when a constant current is drawn:  


  \$ \Delta V = \dfrac{I \times t}{C} \$ 


or  


  \$ C = \dfrac{I \times t}{\Delta V} \$  


Let's further assume that you need 100ms to write the buffer to the SD-card. Then the only 
remaining variable is \$ \Delta V\$. We started with 5V minus 1 Schottky diode drop, giving 4.5V. The minimum voltage drop-out for the MCP1703 is 725mV, so we can go down to 4V, and \$ \Delta V\$ = 0.5V. Then


  \$ C = \dfrac{20mA \times 100ms}{0.5 V} = 4000 \mu F \$


Now the values I used are rough guesstimates, and you'll have to make the calculation with the correct numbers, but the guesstimate indicates that you may not even need the 0.5F supercap after all, though it gives you a serious safety margin. You would for instance have 10s instead of 100ms to flush the buffer to the SD-card.  

(the dropout for the Seiko S812C is only 120mV, so this will double your allowed voltage decrease and hence your available time.)