can a iphone3g be updated still?

I have a Iphone3G with ios 4.2.1. Downloading not so good you have got to have at least ios 4.3 for most apps. People saying there is no more updated for them because too old. Can a iphone3G's ios be updated still? Is it safe and could I be able to use it the same day? plus would be better if using app store and it only pulls up app with your ios, so not having to search so much then start over if not compatible with phone... Thanks