Even when I disable ALL notifications, I still get text message alerts, how do I go about disabling THEM on an iPhone 3G?
It's impossible to completely remove any notification for SMSes prior to iOS 5. The best you could do is choose 'None' as the SMS Tone, and hide message previews.
Since you unable to upgrade to iOS 5, you cannot change nor remove the SMS Message popup box. Under iOS 5 you could change it to the less intrusive banner, or remove any sort of on-screen notification at all.