Of course, you can always brick it, nobody is going to say you can't.
But really it depends on which version of the system menu you update to... it's pretty safe to go up to 4.0. That gets you direct SD access which really makes softmodding easier. If you go up to 4.1, that's just a minor update, but 4.2 is going to stub out all of the known cIOS slots and break any homebrew/hacks that you have in there. That doesn't kill anything permanently, though. You can then restore your trucha bug, use that to drop all of the required cIOS back in, and it will all work again.
This is probably the best guide out there at the moment.
If you decide to go so far as using a hard drive let us know. I have done a lot of research lately into the various cIOS revs and how they affect the various homebrews out there.
Definitely, if you do this, first thing you want to do is install bootmii to boot2 (I am guessing your Wii is old enough for that) and then back up the NAND as suggested above. Doing that will gain you the ability to unbrick your Wii if something goes wrong.