Refer to MyHack (google it!) which has a comprehensive guide. Take in notice that even with retail you'll have issues because of your AMD CPU: it will only work in legacy mode (32-bit), the stock finder won't work etc. If you had a Bulldozer, you could run it flawlessly with full 64-bit app support.

Some people are just lucky, their old MoBos can settle with an upgrade to Bulldozer. Mine, despite being AM3, would not accept it. It's a sin. So i'm at the same boat as you, running Lion 10.7.4 on an outdated Athlon II X2 250 CPU. Anyway, even a 32-bit OSX grants me more computational joy than Win 7 ever gave me.