follow these steps to get to an engineering rom: http://forum.xda-developers.com/showthread.php?t=529019
then follow these steps to get 629 radio/176 spl: http://forum.xda-developers.com/showthread.php?t=605239
take standard precautions - check md5 on all the files, don't be pulling out usb cables while you're flashing and don't be doing anything that might piss your computer off mid flash and you should be fine.
sms can be saved to gmail with sms backup, contacts are sync'd with gmail, you can backup apps with astro or similar. you don't need to touch your sd card so it will remain intact.
you can nandroid your gyd setup but it will be of no use to you unless you revert to a 133 spl and 222 or 322 radio.