Can I buy the 16g unlocked iPhone to use for Straight Talk - will it work?

I'm very interested in buying an iPhone to use with the Straight Talk SIM card. Every time I ask a question about it, my answer says "Buy the $750 iPhone". There's a 16g one for $650, and I'd like to save $100 if it's not vital (who wouldn't?). I was wondering, is there a reason I'd need 32g? Thanks!
http://store.apple.com/...e/iphone4s

Will if you wanna save money get any iphone and unlock it at unlockatt.com

Yes you can if the iphone is unlocked.