Looks like I have a 32GB iPad (really, I had to check)
At the moment here's how it breaks down....
3.4 GB audio
11.6 GB video
1.5 GB photos
2.4 GB apps
0.01 GB books
9.5 GB free
The 1.5GB contains 2,291 photos so I guess a 16GB iPad loaded with a few apps (really, you need Angry Birds) would probably have space for 10 - 15k of your best pics.
The videos are a couple of promo vids + half a season of Ugly Betty and a couple of seasons of Peep Show. There are only 434 songs on there (I don't listen to music on it much). 3 full length books fit in 0.01 GB. Portfolio app full of pics weighs in at 50MB.
16GB is PLENTY for a photographer.
BTW Hasselblad have now launched their app - shoot tethered to a MacBook and pics can pop up on an iPad for the client to rate in real time......