I have a 3G. I jailbroke it so I can use it on T mobile. I try to install apps from the appstore and it says this app not compatible with this phone. It has happend about 12 different times on 12 different apps. Any suggestions would be appreciated. My ios version is 4.2.1
Unfortunately no. Just need to check it's compatibility. I will say, though, that roughly 90% of the App Store apps will require iOS 5.0 or higher... if not 6.0 or higher at this point.
If you'd like to properly utilize those apps you'll, unfortunately, need a newer iPhone.
You'll have to manually find them. There aren't any compiled lists that states the iOS requirements for apps. The minimum now for about 99% of apps in the AppStore is iOS 4.3 and higher, rarely will you find an app below that.
Well in most cases, it would be aimed more towards performance. The 3G's hardware is a bit lacking compared to the 3GS and 4, and especially the 4S and 5, so you'll have some really choppy performance.
Hardly a forced upgrade. The phone came out in June 2008, 4 upgrades to the phone since then. The IOS has been out since November 2010 and has been updaded at least a dozen times since then.