welcome to the forum!!!!!!!
first off lets verify that you have s-off. go to hboot (bootloader) by power off then press and hold power+vol down. at the top of the screen does it say s-off or s-on? also download terminal emulator from the market. load it up and type su if you get # then you are rooted.
ok so i would not use rom manager to start with. rom manager is buggy and has been known to create more problems then it is worth. do everything manually in recovery. also clockwork mod especially version 3 and up use a different script (edify) then what most custom roms are using so you may have problems flashing certain things. clockwork mod 2.6 however uses both edify and amend (most roms use this) same goes for amon ra 2.3. most of us here uses amon ra 2.3(that is what i use). you can get them along with instructions in ref#1 of my sig.
ok so here are the steps to manually flash a rom
1.MAKE A NANDROID BACKUP. THEY WILL BE YOUR FRIEND.
2.wipe dalvik cache, cache, and system.
3.i do an extra step and use Calkulin's_FORMAT_ALL.zip
. it basically wipes system and both caches for you. i like a really clean slate. this step is optional
4.select install .zip from sd card and find and select your rom
now the first boot should take anywhere from a few minutes to no more then 10 minutes. it needs time to build dalvik cache.
hope that helps