IPhone 3g - Upgrade help

[Anarion1191]

Hello guys,
I have an IPhone 3g with 3.1.2 firmware right now, jailbroken and unlocked using blackra1n. This recently, I don't know what happened to cydia, everytime I want to start it, it crashes. So, I decided I better upgrade to a +4 version(4.1.2 or anything possible), given that I need an application which runs on iOS4+ only.

Which method should I choose to upgrade and then jailbreak/unlock again? I have no access to a Mac, only windows 7 and Linux.

and here's the modem version in case it is needed:
Modem Firmware: 05.11.07

Thanks in advance

[iPittsburgh]

Use sn0wbreeze to make a custom firmware so that you can update and keep the ability to unlock. Here's a guide to follow Sn0wbreeze 2.1 iOS 4.1 Jailbreak [Custom Firmware Windows] | Redmond Pie Just be aware that iOS4+ tends to slow down an iPhone 3G.