Well, there are many ways to root your phone.
The a reall easy method, and the one I use is the OCLF 2.0 app in the market. All you do is download it, launch it and select root. It will do some stuff and spit some words on the screen, then tell you to reboot the phone in recovery mode.
This is done buy shutting off the phone. Hold down the Vol up AND down buttons, then push and hold the power button. The white AT&T screen will come up (continue to hold the buttons) and dissappear, then re-appear. Once it reappears let go of the power button. It should load into recovery mode. Use the volume up/down to navigate to "reinstall packages" and push the power button to select it. Your phone will do some more stuff, and then re-boot. Once it's booted, rooting should be successful. You will have an app in the drawer calles super user, that will confirm it worked.
The back up method I use is TiBu (paid version) to back up apps and app data. (Its stores that info on an internet server) so you dont actually need to put it on your PC or anything.
I use ROM manager (clockwork) to make a back up of my current rom. (then load that to my PC). I also use ROM manager to flash new custom ROMS.
As far as restoring, I usually use ODIN 1 click, which will bring your phone back to its "out of box" state. I also use ODIN to roll back before I re-flash a new ROM.
The benifits of a custom ROM are, I am currently using Froyo (Android 2.2) before it is even released for our phone. It also offers more features, tweeks and customizations, root and other things. The down sides are, they are un-official. The Cog. froyo beta (although pretty good) is built on a beta release of Froyo and done completely with out even the source code (Sammy hasn't released it yet). So sometimes there are minor bugs or issues. Some features are experimental and sometimes down work propperly. But, when there is a wide spread issue with it, they are pretty quick about putting out a fix (which means you also have to go thru the flashing/recovery process again tho). It isn't for everyone and requires some patience and common scence, but if you like to tinker, experiment, and live a little on the dark side, it can be fun and enjoyable. I really love having flash on my phone, for example, and it is very fast now too.
If you root now, you will have to re-root when you upgrade to 2.2. It will unroot you AND you should probably inroot before you upgrade anyway, just to be safe.
It's YOUR phone, so have fun with it. Just be responisble and have a good understanding of what you are doing and how to do it, before you do it
The XDA forum is a good source of custom ROMs and lots of info about them. This forum here, there is a handful of helpful people who know a bit about that stuff and willing to share info too.