ROM stands for Read Only Memory and in this case refers to the phone's internal memory, where the android OS is stored and so on. When you apply a new ROM you're just wiping the Read Only Memory of your phone and replacing it with a fresh copy that contains newer firmware. So don't think of the ROMs HTC release as being a feature of your phone you can update, they're just containers for new versions of firmware basically.
So like brykins says, the current ROMs available are simply going to give you a slightly tweaked version of Android 1.5. The 2.1 ROMs will replace the 1.5 firmware with 2.1.