Yeah, it will wipe them off your SD card. At least, the default way is.

Anyway: For power kernel, I would wait a little while - the current uboot in the repositories flashes (from onboard the device, so you don't notice it, but that's what it does) a stock PR1.3 kernel, with uboot attached to the beginning of it.

There's a power kernel, + PR1.3 patches, + uboot, available here:
http://talk.maemo.org/showthread.php?t=64921 (On page 2 there's a link to it in Matan's post, with some instructions on how to best make it work on page 3 and 4.)

You can basically download it, and flash it (again, in a technical sense, not 'flash' it in the sense that you kill all your applications and have to restore everything), all from the device.

BUT, I would suggest that people wait until Titan produces his own power kernel for PR1.3 - it will probably also come with uboot (though I don't have any sources, I'm just making an educated guess), or if it doesn't, I'm sure someone will stick it in extras-devel as well. This way you can have power kernel on the actual NAND, and MeeGo and Nitdroid somewhere else. And to hell with the stock one. One the other hand, if you want the stock one, the above already works fine for you for now.

As for flashing your SD card with MeeGo / anything else... Honestly, if you want to multiboot to anything more than two OSs, you'll probably need to make some complicated mix of partitions to make everything work perfectly - and that means that you'll only get to flash one OS image to your SD card - beyond that, and you have to manually partition the rest of the card / internal storage. (At the moment anyway. I am optimistic there will be a tool that lets us do this in a end-user-friendly way eventually.)