I purchased a used iPhone 3G that I want to use with my T-mobile plan. TinyUmbrella shows that the phone is currently running firmware 3.1.3, with baseband 0.5.12.01-6.4_MS32.

I think the tutorial at iClarified - iPhone - How to Jailbreak Your iPhone 3G on OS 3.1.2, 3.1.3 Using RedSn0w (Windows) will work for me, but I have a couple of questions. First, the firmware downloaded from the iClarified site is 3.1.2. Will this work running agains 3.1.3?

I found a version 3.1.3 ipsw at Greenpois0n.us. Assuming I can trust this site, would I be better off with that file? (I couldn't find a link to greenpois0n on these forums, and there is also a greenpois0n.com. Are either of these legit?)

I would also like to upgrade to firmware 4.2.1. I can't run pwnage, since I have no access to a Mac, and winpawn seems to be quite out of date. I downloaded the 4.2.1 ipsw from iClarified. If I use shift-restore from iTunes, can I load 4.2.1 or will that also update baseband? And is there any way to tell in advance what baseband that ipsw will load?

tl;dr: What is the best procedure to jailbreak and update this phone to 4.2.1 without updating baseband?

Oops, I've done a little further reading and 4.2.1 without baseband update might be out of reach.

theiphonewiki lists 4.02 as being available with an unlockable baseband. Can I just load this using the iTunes shift-restore method, then jailbreak with redsn0w?

Couple of things:

1) Do you have an SHSH on file for any of the 4.x versions you're discussing? It's worth mention that without them, the best you'll be able to do is 3.1.2.
2) Do not download .IPSW files from anywhere on the internet except for Apple's site. You can easily turn a legitimate .IPSW from Apple's d/l servers into a custom .IPSW, and it's always safer to make sure you're doing the legwork so that you know the .IPSW is safe.
3) Your current baseband would be handy for us to know.

First off all NEVER restore a stock firmware because will update your baseband, and you will not be able to unlock normally. You should download a 4.2.1 stock firmware from apple servers (now almost all website that are providing iphone firmware are redirecting to apple so its ok, can try felixbruns.de). After that download from ih8snow.com the sn0wbreeze v2.2.1 and create a custom firmware with the stock firmware you downloaded before. Then put the phone to pwaned dfu and restore the custom firmware, be carefull do not restore the stock firmware (the custom firmwares names will begin with snowbreeze).Good luck

I went ahead and used the above linked walkthrough to unlock the phone running 3.1.2, keeping the baseband as listed in the OP. It is working fine now. A couple of times it has forgot that it is on the T-Mobile network, but a reboot fixes that every time. The iClarified walkthrough is mostly straightforward, though it coule be a little more precise in a few places. For instance, it says to use redsn0w .9.2, but not there nor anywhere else did I find it in writing that any newer version will not work. Most software I've dealt with usually includes some backward compatibility, so this was a surprise.

It really is quite a jumble of information to sort through. It is not uncommon to search these forums, find a half dozen threads with a similar sounding question, only to find that the only answer contained in them is "this has been asked and answered before, please search the forums." So I really appreciate that you both took the time to answer my question. Thanks.

As for the SHSH files, I am not 100% certain on this. I think tinyumbrella was showing me that it had SHSH files for 4.2.1 and 4.1, but I'm not sure I understand what it is showing me. How can I verify that these are for my phone?

I have the 4.2.1 from apple, and sn0wbreeze 2.2.1. I ran the program, but it wasn't completely clear to me what it was doing. I saw an option for custom baseband. But there doesn't appear to be any way to verify what baseband is on the resulting IPSW file, so I was reluctant proceed. I was considering trying to run OSx in virtualbox so I can try it with pwnage, but I may take another look at sn0wbreeze tonight as well.

snowbreeze build a custom firmware that does not update your baseband so your iphone will have the same baseband after restoring. Any of the 3 options in snowbreeze builds a custom firmware without baseband update, and just select simple mode will make a custom firmware jailbroken and hactivated.If you do not want to hactivate and have a official sim to activate use expert mode and select deactivate.To update to 4.2.1 you do not need shsh, other shsh saved will show in cydia on the home screen.Or open tiny umbrella and select save shsh. If you appreciate my help please press the Thank you button