I preferred to take the card 16 GB for to store downloaded games (including PSP and PS One games), backups, applications, movies, photos and music, 8 GB would be quickly filled. For example, a game like Uncharted Golden Abyss weighs 3.3 GB, 1.6 GB WipEout 2048, and Metal Gear Solid (PS One) 730 MB.
The memory card is required to use the PS Vita (which, alas, has no internal memory), it is regrettable that it is not supplied with the console, especially as its price is quite high.