We're going with doing it manually because its actually easier because issues always arise with ROM Manager
Once rooted (which I think you are) download ROM Manager and open it up and go to "Back up current rom" and back your current up. Then at the top select "Flash recovery" pick the Ascend out of the list and flash the recovery. (I suggest hitting the flash button 3 times to make sure it flashes)
Then once the recovery is flashed, pick a rom download it and put it on the root of your sd card. Download the gapps too and put them on the root of your sd card. You can find CM7.2 here:
CM7-Ascend - Prepaid Android and heres the latest gapps:
gapps-gb-20110828-signed 7.2.zip
then once they are on the root of your sd card (root meaning not in a folder) turn the phone of and while holding the send key and volume up key tap the power. You will then go into clockwork recovery.
Go to "wipe data/factory reset" and wipe it, then to wipe cache partition and wipe it, then to advanced and select wipe dalvik cache and wipe it. Then go to mounts and storage and format everythinng except the SD card. If you format the sd card it will erase everything on your card.
Then go to install zip from sd card then to choose zip from sd card and select the rom zip file and flash it. Once that is done go back to choose zip from sd card and flash the gapps. Once those are done hit your back key a couple times so you are on the main screen and go back to advanced and select fix permissions then once that is done go to "reboot system now" When your phone reboots you will be on cm7 and you will have to put all your gmail info in again like you did when you first got the phone.
then youre done. Enjoy