Well some say 3.1.3 works best on an iPhone 3G but its really up to you. If you feel its running abnormally slow then you may want to consider downgrading to 3.1.3 but I think the only trouble you may have when downgrading is that you have the iPad baseband and may need to use a custom firmware.
3.1.3 is the best for iPhone 3G. But if you want to be able to stay on 4.1, disabling multitasking and wallpaper should improve performance very significantly. Also removing unused language packages will help a lot for a speed boost.
No, you would have to use a combination of mobileterminal and SSH (or iFile if you've got it) to run the code for removing the language packs. Unless you want to manually SSH in and delete all 3000 of them one by one lol.
YouTube it, it's easy. If you search "remove unease language packs" and watch the vid by dr jailbreak, it will help you. He also has included download links to the language pack files that will remove the ones you don't want.
Up to you. I don't mess with those. One time it crashed my phone
Then again I probably deleted something incorrectly. You should find a launch daemon disabler app on Cydia for that. Be sure it works with your fw though.
I use SBSettings, OpenSSH, Backgrounder+Multifl0w (for multitasking), MyOS to disable native multitasking, Bytafont, mobileterminal, iFile, My3G, MyWi, NoSpot (turns off spotlight search), Safari download manager, YourTube 2, unlimtones, probably a ton more lol. But that's most of it. Oh and notified pro for alerts. Some are paid apps but they're worth it.