Possible solution regarding the crashing of FTB launcher when attempting to start the new Horizons pack:
1. Back up anything minecraft-related you don't want to lose
2. Delete everything minecraft related -- all of the modpack folders and your base .minecraft
3. Download the updated minecraft launcher and FTB launcher, make sure you have most recent java
4. Run the regular unmodded mc launcher and have it use version 1.6.4 so that it installs a fresh copy of mc (under profile settings), quit minecraft after it downloads and starts
5. Right-click on the FTB launcher icon, go to the compatibility tab, and select the options for the launcher to run in 'windows 7 compatibility mode' AND 'run as admin' if you have admin rights
6. Start the FTB launcher and download Horizons. It should work now.
Possible problems:
1. You'll still get the wall of red text about missing textures, but Jadedcat seemed to say that this was expected
2. Game might still crash for other reasons -- read the error log... this might be memory-related (using x64 and only 1GB of mem seems to be more problematic), but it seems to make it past the launcher after above steps
Why I think this might be happening: (I'm not a programmer though, just speculation)
All previous versions of FTB modpacks relied on 1.5.2 or earlier, and the new one is 1.6.4, which used the new version of the vanilla mc launcher. Is it possible that upgrading the mc version to both a higher version and one that uses a new version of the launcher is causing compatibility issues between ftb and vanilla mc? Considering that my game wouldn't even launch until I deleted everything and only downloaded version 1.6.4, this seems somewhat more likely.
Things I have not tried to do yet since getting my game to launch:
1. Changing which mods are used via the launcher options
2. Downloading and running any of the other modpacks available in the launcher
3. Placing any complex series of blocks in-game. I was, however, able to go from failing to launch to starting the game and creating a new world.
Hope this helps.