Well, that depends on what you intend to do. If you only want to do the occasional web browsing to say look something up, or check your favorite website occasionally, then 500MB should be fine.
If you're like me though, who uses my phone with the web more than my actual computer, then you should go with 2GB. I have the 6GB (not available anymore) plan and do well over 1 GB. I do regular web browsing, download the occasional app from the App Store, and watch the odd Youtube video. Most people will probably be fine with 500 MB, regular users I would think should have 1 GB and heavy users over 1GB. Keep in mind that if you are going to do streaming Internet Radio, or say upload videos should that be a feature in the New iPhone, your data use will probably spike quickly.
So think of what you want to use it for and base your choice on that. Hope this helps.
Also, don't buy now if you can avoid it. New iPhones should be on the way very soon. Apple's Marketing chief has even said the June period is roughly the time when they do annual upgrades. Plus lots of other things point to an upgraded phone. Wait if you can.
