as for trying ROM's it is actually really easy.
since you are rooted the first thing i would do is download titanium backup, and do a batch backup of your user apps and their data (makes switching ROM's so much easier)
DO NOT BACKUP OR RESTORE SYSTEM APPS EVER!!!
check out the ROMs on here or over at rootzwiki.com
CyanogenMod is the most popular ROM out there. it is FAST, customizable, and has no sense, but there are quirks with it, just like any other AOSP based ROM.
Das Bamf is the most popular sense based ROM with some really nice tweaks.
the second thing i would do, is grab the latest OTA radio and flash it through hboot. most, if not all Gingerbread ROMs can use this radio. and i have had the best results using it.
and to answer your question about the unlimited data, it will carry over no problem to 4G.
if you have any questions or anything, you can contact me through this site. i am a "veteran" flasher...been doing it for about 2 years on various phones, various ROMs totaling in the thousands it seems like...lol