And as for iPhone 3GS with older bootrom, keep your hand of this release if planning to update to the current version of iOS firmware, but waiting the upcoming redsn0w version to upgrade the baseband firmware.
And remember this PWangeTool 4.1.3 currently for Mac platform, therefore it won't work for Windows, at least currently, until iPhone Dev Team comes out that work with your OS.
Download links for PwanageTool 4.1.3 on The Pirate Bay:
Or use the following mirror links if your country blocks The Pirate Bay site:
Update: If you are a Windows users (also Mac Os users), you can update iOS 4.2.1 On iPhone 3G, 3GS Baseband To 06.15.00 For Windows, Mac using Redsn0w 0.9.6b5
After you have completed the steps to update your existing baseband on that page, then continue your steps to unlock iOS 4.2.1 on iPhone 3GS, 3G with Ultrasn0w 2.1 so you can use any carrier.