Just install z4root.apk and you should be fine. And you can always un-root.

http://forum.xda-developers.com/showthread.php?t=833953

 

I'm pretty sure rooting doesn't even void your warranty. I had an AT&T rep telling me he rooted a friends phone in store. Of course I think flashing a ROM is another story. I've never really worried much about warranties after the first 6mo.

 

Have you viewed this?

Root - CapFAQ

 

Cognition 3.04 is the only ROM I've flashed (3 different upgrades of Cog no problems with the flash). I'm very happy with it. Although Serendipity 5.5 is another excellent ROM that has lots of fans. If I flash anything else it would be that. These both upgrade your phone to Firmware 2.2 Froyo. I think the stock Froyo from Samsung would be a downgrade once you get one of these flashed. Thats when Samsung finally comes out with it. And you can't expect them to fix bugs like the developers of the ROMs do. Samsung took 7months to even release the upcoming 2.2(Late Feb maybe). If they even do release it.

 

Cognition is generally accepted as the most stable rom. I was on cognition 3, it was perfectly stable. I had tried serendipity 4, but I got some crashes, but serendipity 5 has been perfectly stable for me and has a great gingerbread theme so I would recommend serendipity 5. You won't have to do anything to get froyo after flashing it, the major reason to flash roms is because they give you froyo along with the theme, apps that require root, tethering etc.. On the other hand, if you want to flash back to stock U.S. froyo whenever its released, if you want to do it OTA, you may have to flash back to stock.