Step 1
First you have to root yr Atrix. I rooted mine with Gingerbreak - go to Android Market, install 4shared. Search for Gingerbreak, download that and follow the prompts, eventually it will install Superuser whick will allow all these wonderful custom procedures to work in yr Atrix. Next >>

Step 2
Pls refer to the following 5 youtube tutorials by TheSmokingAndroid, who has been extremely thorough, supportive and helpful in upgrading Motorola Atrix. You MUST strictly follow the order ((i personally view the videos few times, research some of the recommended custom ROMS and kernels before i run the upgrade)) :

1. Motorola Atrix: Unlock & Upgrade to GB - Part 1
2. Motorola Atrix: Unlock & Upgrade to GB - Part 2
3. Motorola Atrix: Unlock & Upgrade to GB - Flashing CWM
4. Motorola Atrix: Unlock & Upgrade to GB - Flashing (*GB 2.3.4) - Part 1
5. Motorola Atrix: Unlock & Upgrade to GB - Flashing (*GB 2.3.4) - Part 2

* - You can choose any Custom ROM i.e AURA,Alien,HomeBase & CM7
Good luck!!