I have an 8GB which seems to be fine for now. The one game I've downloaded, Sangoku Musou Next, takes up around 1.4GB. If you're not going to be buying games from PSN, 4GB should be fine for a while.
That said, as soon as 32GB cards are available somewhere, I'm picking one up.
On the fence about importing Gravity Daze. I know it's definitely coming to the US and so spending $80-100 importing it just to have it at launch with jp voices (seems like an anime game so would rather play with jp voice track) doesn't seem worth it when I can get it for $40 a few months later.
Gravity Daze is coming out in the States as Gravity Rush, and it's listed as a launch title...one week after the Japanese release.
The characters also do not speak Japanese in-game. It's a made-up language and sounds like gibberish.
Also I want to buy Power Smash 4 after DC's raving about it but WTF it's not listed for US release on Amazon? Hopefully they'll add it later, I'd be really surprised if a tennis game that takes no effort to localize wouldn't come over.
There are a few games that either aren't listed or don't have solid dates. Dynasty Warriors Next is like this too.