Best way to Jailbreak/Unlock iPhone 3G running 4.2.1

[elayman]

So I've been spending the past 2 hours researching on how to jailbreak and unlock my iphone untethered. I've read about redsn0w and ultrasn0w but I'm a bit confused.

I don't mind downgrading (if it's even possible) to another firmware but I tried putting it in DFU mode and installing 4.0 but that had an error.. so can someone give me a good write-up if this is necessary for my needs.

First of all, what are SHSH blobs? If I'm already at 4.2.1 should I bother using TinyUmbrella to do this or is it useless since I can't downgrade?

Also, are there even any untethered jailbreaks that allow me to unlock my phone to use on T-Mobile?

Please help. Here is what I was reading, however it says it's a tethered jailbreak and I don't want that.
Redsn0w 0.9.6rc8 Download Available
I've read about a different redsn0w version that is in beta version that allows untethered, but it tells me to use the iPad baseband.. and I'm not sure I want to do it if it's irreversible. Can I just downgrade to another version and unlock?

Here is what I plan on using to unlock:
Ultrasn0w Unlock for iOS 4.2.1 Download Available

[i.Annie]

1) To unlock, you should check to see if you can downgrade your baseband. Use F0recast from iH8sn0w to see if you have bootloader 5.8. If you do, skip this next part. If you don't, you'll have to upgrade your baseband to the ipad baseband of 6.15.00. This has its risks, you may lose GPS function.

2) Depending on which firmware you want to be on, you will use different jailbreak tools. If you need to upgrade your baseband, your only option is to use Redsn0w 0.9.6rc8.

3) If you'd like to have the ability to unlock and you need to use the baseband upgrade, use Redsn0w and install the iPad baseband. Simply downgrade to 3.1.3 for best performance on your device.

4) If you go to 3.1.3, you may use Spirit, Redsn0w 0.9.4, or Jailbreakme.com. Jailbreakme is by far the easiest to use as it does not require a computer. To downgrade, simply get a stock 3.1.3 ipsw and shift+restore to it. If you encounter error 1015, simply exit recovery mode with tinyumbrella.

5) If you have bootloader 5.8, you may downgrade the baseband. Simply restore to a stock 3.1.3 and jb with any of the above listed tools, and install Fuzzyband. Let it downgrade the baseband, then install Ultrasn0w.

6) If you don't want to downgrade to 3.1.3, stay on 4.2.1, jailbreak with redsn0w 0.9.6rc8 but do not upgrade the baseband if you have bl 5.8, install Fuzzyband from Cydia, downgrade the baseband, and then unlock with Ultrasn0w.