8 GB. It's a pretty simple choice with double the memory. It will be much more worth it in the long run to spend the extra hundred and have much more space.
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Picked up a 32GB iPhone 3GS in Black on Friday, June 26th. Lovin' it!
There were already 2 polls for this. You've just reduced the number of answers we can give. I go with option #3. "I'll try for an 8gig, but if they run out, i may buy the 4gig." lol