I'm getting ready to buy the 64gb 3G version when it's in stores next week. But that's only to make sure I'm future-proof. Right now, the 16gb can handle everything you need, most likely.
At first I worried about putting on all of my iTunes songs and videos, etc. But I plan on getting a Rhapsody subscription for $10 a month, which gives you streaming access to pretty much every song out there. It takes up next to no space, and with 3G access, it means having access to an unlimited amount of music no matter where you are. Which is great. With iPad OS 4.0, there will be support for background applications, which will make Rhapsody on par with the iPod app.
And for video, I have a Netflix subscription that I use for my Xbox and tv-connected Mac Mini (I dumped cable months ago, it's great). With the Netflix app available for free, that gives me access to a ton of movies and tv shows, and once Hulu becomes available in late May for an expected $10/month, then I'll have access to all the new shows too. It's way cheaper than iTunes, and it's legal. And if you must watch your downloaded videos, then get
Air Video, which converts the videos on the fly and streams it over your network to your iPad.
So why get the 64gb? When I bought my iPhone, instead of the 32gb, I opted for the 16gb. It was fine at the time. I couldn't anticipate needing much more, since games and apps were relatively small a year ago. Then I downloaded Navigon, which is pretty huge, and a ton of apps, including games that are several hundred mb in size. I can't see why you would need 64gb now in an iPad, but in a year or so, it'll be obvious.
Just my 2 cents.