yes you can flash ROMs. most of the bigger ROMs have provisioning programs that run after first boot. just pick your carrier and it does the rest. some require adding a couple of cabs to get messaging working.
1. unlock your phone here:
http://forum.ppcgeeks.com/showthread.php?t=93252
make sure to follow instructions COMPLETELY. turn off antivirus, turn off firewall while doing this or it won't work.
2. once you are unlocked, then you can flash any cdma ROM on this forum. I would try a mighty mike or energy or Mr. X ROM the first time. there is a reason they are most popular. Make sure you read the first few posts of each ROM. It will tell you exactly what you need to do, like flash, then run carrier provisioning, then install any fixes. as far as flashing a ROM, usually it is just an exe that you download to your computer then run the exe while you are synced to your computer.
http://forum.ppcgeeks.com/forumdisplay.php?f=98
that is the forum location for cdma ROMs. stay away from GSM Roms and GSM radios, they can kill your phone.
really, flashing a ROM is very easy. just read a thread a couple of times so you understand what you are doing. once you see what is out there, then you can start looking for a ROM that fits what you want. personally I hate all forms of sense/tf3d and use mobile shell in mine.