I've read both 16GB and 32GB will be available, plus it will be able to take the new 128GB microSD card.
I also know I read somewhere that there's like 8 or 9 different models, that vary by country and carrier.
Addendum: Here's one of spec sheets I saw:
Models:
Samsung SM-G900F/SM-G900H - for Europe
Samsung SM-G900I - for Asia
Samsung SM-G900K/G900L/G900S - for Korea
Samsung SM-G900M - for Vodafone
Samsung SM-G900A - for AT&T
Samsung SM-G900T - for T-Mobile
Samsung SM-G900V - for Verizon
Samsung SM-G900R4 - for US Cellular
Samsung SM-G900P - for Sprint
Memory:
Card slot microSD, up to 128 GB
Internal 16/32 GB storage, 2 GB RAM
http://www.gsmarena.com/samsung_galaxy_s5-6033.php