Follow this way to make iPhone 3G with iOS 4.2.1 Installed.
BUT, we must Jail-break this device first. JAIL BREAK iPhone 3G iOS 4.2.1 you can learn from this link.
After your iphone jailbroken, you`ll see Cydia icon on your homescreen.
Now click Cydia, and let Cydia sets up the filesystem, and then reboot device.
Install Tools from Cydia
Click Cydia > Manage > Source
Click EDIT > ADD
and, fill following repo
1. stop unnecessary Daemons from slowing our device.
Search Cydia Tweak called Daemon Tools from repo.insanelyi.com
2. Install Springtomize from the sinfuliphone repo.
This app will remove features that will speed up your iphone a lot.
Once Springtomize is installed, Click icon Springtomize
Choose Settings > Springtomize and apply these tweaks
General – Disable Spotlight –> ON
Icons – Hide Shadow –> ON
Icons – Disable Jittering –> ON
Dock – Hide Icons Reflection –> ON
Lockscreen – Slider – Custom Sting –> ON
Folders – Remove Animation –> ON
App Switcher – Start Editing –> ON
Animations – Custom Duration –> ON
After you have done this, Click respring.
NOW, remove all of the Spotlight indexing options in Settings > Spotlight Search OFF, and to turn MMS off.
3. Install SBSetting
see, iPhone 3G with iOS 4.2.1 now runs faster 🙂