Best way to upgrade my Jailbroken 3GS running iOS 3.1.2 to iOS 4.2.x?

[cashonly]

OK,

Now that it appears that with ver 4.2, iOS doesn't bring a 3GS to a snails pace so I want to upgrade my 3GS to the latest v4.2 iOS and keep my jailbreak. Lock is not a concern as I use AT&T and they already have a contract lock on me.

Can someone point me to instructions on how to do this and get an untethered Jailbreak? (I only have a Windows PC).

Here are the detailed Specs on my iPhone
iPhone 3GS
iOS: 3.1.2 Build 7D11
Model: MC138LL
Firmware: 05.11.07
Carrier: AT&T

Jailbroke Apps that I really want to keep using:
MyWi
Backgrounder
SBSettings
Cydia
3G Unrestrictor
biteSMS


Thx In Advance,

Cash

[Simon]

You should download idetector and find out what bootrom you have since new bootrom 3GS is a tethered jailbreak on 4.2.1 right now.