iPhone 3G running iOS4.0 (Upgrade or Downgrade?)

[D.Schrute]

I have an iPhone 3G running iOS 4.0 with baseband 05.13.04, which was unlocked using redsnow 0.9.5b5-4 and unlocked using ultrasnow. The phone has been running very slowly. So I need a fix, do I downgrade to iOS 3 or move up to 4.1 or 4.2 if it is possible. I need to be able to keep the unlock and preferably an untethered jailbreak.
I don't know much jailbreaking terminology so please go easy on me and help me with guides.
Thanks!

[iYeow]

You can go to IOS 4.1 or 4.2.1 . Run redsn0w to put you into dfu mode .
Open up itunes : shift + restore to this custom fw of 4.1 or 4.2.1 ( in your PM box)

[D.Schrute]

Yes I understand, but will this allow me to unlock it?