For the new bootrom you have two options:
1) GeekGrade's IPSWs (I'm afraid I can't link them here because they are a copyright violation). You put your device into pwned DFU and restore using those with iTunes.
2) Make your own IPSWs using someone else's SHSH with iFaith (here's a Google Drive full of them:
https://drive.google.com/drive/u/0/folders/0ByxMOiAf78kITnB2MEVkU3J4aGs).
The main difference is that the GeekGrade IPSWs automatically prevent the LLB from being verified by keeping the flashed version from a working untethered iOS installations. If you make your own IPSWs, and don't patch manually, you will need to install Insomnia from Cydia to prevent your device from sleeping, and this will drain your battery.
If you feel up for it, I can write a tutorial on how to modify the IPSW to prevent this problem.
Now, if you have an old bootrom, you can restore to any iOS untethered. Make a custom IPSW with sn0wbreeze (bundle it with Cydia, don't make a signed IPSW), put it in pwned DFU, and restore in iTunes.
EDIT: Things have changed a bit since this reply. The new bootrom 3GS has a full untethered bootrom exploit now, called Alloc8 (
https://www.theiphonewiki.com/wiki/Alloc8_Exploit). You can now downgrade untethered to any iOS.