Updating iphone 3g firmware

Hello

I have a jailbroken and unlocked iphone 3g running on 3.1.3 with baseband 05.12.01. It runs on T-Mobile USA. It runs fine right now but I want to upgrade to iOS 4.x. But Im not sure which one should I upgrade to. I have heard that upgrading to 4.0.1 is pretty bad on iphone 3g and 4.1 is a bit better but wont my baseband also get upgraded with 4.1 so I wont be able to unlock? Is 4.0.1 safer in terms of baseband? Which one would you guys recommend? Also Im not sure how to begin the process of upgrading to a higher iOS without losing my jailbreak. I tried looking at custom IPSW but I dont feel safe downloading a pre-made custom ipsw (I dont have a MAC so I cant make it myself). Also where do you guys recommend is the best place to get any ipsw?

PS: Tinyumbrella says I have 4.0, 4.0.1,4.0.2,4.1,4.2.1 saved

We answer this question a couple of times a week, please search more next time before posting a duplicate thread.

Stay away from iOS 4.0, 4.0.1 or 4.0.2 - they are very slow on the iPhone 3G.

Use only custom IPSW to restore - we recommend Pwnagetool. As you don't have a mac, try installing OS X in a virtual machine (there are links elsewhere on hackint0sh explaining how to do this with the free VirtualBox from Sun) - guide for pwnagetool iClarified - iPhone - How to Jailbreak and Unlock Your iPhone 3G Using PwnageTool (Mac) [4.2.1] - it is very important to say NO at step 6 in that guide.

It is against the forum rules to provide links to custom IPSW. Don't download a pre-made custom IPSW, they can't be trusted.

Or use sn0wbreeze at your own risk. We refuse to support sn0wbreeze here.

Use SAM for legitimate activation, otherwise you risk poor battery life.

Thanks a lot for your help. I should have done more research, and I will definitely do it next time. I read somewhere that I can restore iphone 3g back to 3.1.3 even though I dont have the SHSH saved for it in tiny umbrella, I just want to confirm that with you before I go through the virtual machine option.

Yes you can restore back to 3.1.3 (but you can't downgrade baseband) without a SHSH on the iPhone3G