Need help upgrading unlocked iPhone 3G

[HappyBunny]

Ok, I am lost in all the basebands and versions and everything. Please help this confused girl.

I have an iPhone 3G 8MB using 3.0 firmware. It is unlocked and jailbroken and I am using it on T-Mobile. I would like to upgrade to at least 3.1, as some apps do not run on 3.0 and I am also experiencing some odd little behaviors from some apps, which lead me to think a fresh install is overdue. I believe it was originally unlocked with Redsn0w but am not certain. I have read that 4.0, 4.1 will really slow this phone down.

So my question is, what OS should I install? Where can I find anything other than the latest (which of course is on iTunes). And does it matter that I already have a jailbroken phone, in other words, can I run into problems and brick my phone because I choose the wrong update or version or whatever? I actually dont care about thejailbreak, I only buy and use apps from the Apple Store, but I do need to use the phone on T-Mobile.

Thank you so much--I am pretty techie but the amount of minutiae that one needs to know to do the upgrades has overwhelmed me!

[iYeow]

My opinion, go with 3.1.3 for now.
All you need to do is Do a Shift + Restore to 3.1.3 ipsw which you can download from here : iClarified - iPhone - Where To Download iPhone Firmware Files From
You will need to download 3.1.2 ipsw as well onto your desktop.
Now download Redsn0w 0.9.4 from here : ModMyi Forums - Downloads - redsn0w Windows 0.9.4
Run Redsn0w to jailbreak but point to 3.1.2.
Unlock again with Ultrasn0w.

[Gigglingoth]

Most of the apps I have or want now require 4.0 or above. I also have the iPhone 3G running 3.0.1 which is jailbroke and unlocked. Can someone please tell me how to upgrade to 4.0? I've never had a problem with these processes before but I too am at a loss when it comes to the basebands. The last thing I want to do is lose my unlock