So you rooted your droid. Did anyone tell you that you'll not ever receive an official update ever again unless you undo that process? Did they warn you that you've voided your warranty? And unless you unroot you'll have to rely on whoever made your custom ROM to get around to porting the updated code from Google to their ROM, testing it themselves (assuming they do at all), and releasing it. You then have to take the risk of installing it, hoping that you don't brick your phone in the process (that voided warranty might sting a bit if it does), and hoping that their testing fixed any fatal bugs they may have included, and that any security holes Google kept quiet about have also been fixed...
So, to answer your questions:
There are three ways to update your phone:Unroot, install the official ROM, and wait for the official update to appear that is currently being rolled out.Install a different custom ROM that's more up to date.Install an updated version of your current custom ROM.
Is your current ROM up to date? That depends on many factors. How old is it? What version it reports (check in the system information, it'll be 2.0, 2.0.1, or 2.1). What stuff the creator took from 2.1 and added to your 2.0 ROM. Check their website or ask around, but if it doesn't report as 2.1 it probably doesn't have all of the stuff from 2.1.
If you unroot and let the 2.1 update install, you'll be unrooted, so you'll lose any benefit you may have had when you rooted. If you could tell us what those reasons for rooting were then we'll be able to tell you if you'll lose those abilities if you unroot. If you need to be rooted, then I'd wait for an update to your custom ROM to be made.
You will not receive a notification of a new official ROM being released, nor will it install (official updates can only be applied to certain official versions of Android, and your custom ROM isn't one of them). But, some custom ROMs have their own updating systems which may notify you and offer to install the new ROM for you.
To get the best performance out of your Droid, you need the most up to date and optimized ROM that's available. This may be the official ROM, or it may be one of the custom ROMs available - it just depends what's in them. In general, mature custom ROMs offer better performance and more features earlier than official ROMs.