Automatically selection of voltage sources?
Providing a link to the data sheet usually helps muchly. As, often, does reading said data sheet. IF in spec then a capacitor at regulator input may be a good idea.

Datasheet here

Page 12 says:

Vd Dropout voltage typical / maximum
IO = 100 mA ... 1.00 /  1.1
IO = 500 mA ... 1.05 /  1.15
IO = 800 mA ... 1.10 / 1.2  

With a 1n4007 at nominally 0.6V drop you have 5-0.6 = 4.4V IF the ISP nominal 5V is actually >= 5V.
Acceptable dropout voltage is then 5-0.6-3.3 = 1.1V.
This is exceeded in above table only at 500 mA 800 mA worst case.
Using a Schottky diode for D2 puts Vin_regulator in spec for all currents up to 800 mA.  

If the ISP 5V is sourced fron a "5V" USB port and there is potentially connector and wiring drop as well worst-case-V_ISP may be say 4.8V and maybe lower. If so, deduct this from the allowed regulator 'headroom'. 

Read the data sheet to be sure all other requirements are met.