Updating iphone 3g firmware
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