Vault boy Eddie said:
The deed is done, canceled MW2, preordered Mass Effect 2 Collector's Edition and am now installing Borderlands! F*CK YOU IW!!, F*CK YOU WHARE!! AND F*UCK YOU DORPHIN!!!!!
I will go with some other people here and recommend you play ME 1 first. You'll get a better idea of the AMAZING story, and ME 2 has a neat feature where it will remember your choices from ME 1 and tailor the game to keep the two consistent.
For example, if any of your co-horts from ME 1 die during your playthrough there, they will still be dead in ME 2.