Definitely play the first one.
Personally I think if you play ME2 first, you may find it harder to transition to ME1. In sequence, the transition is not difficult to adjust to. Both games just play differently.
ME1 certainly sets the scale of grandeur for the story. Only KotOR surpasses that for me.
ME1 is driven by the main story, which is nothing short of epic.
Only gripe I have with ME2 (apart from the HUD and key mapping) is the modular nature of it. Don't get me wrong, I'm enjoying the game, but it feels very broken up with all the 'Mission Complete' screens for one.
Another very annoying feature is entering /leaving the Normandy. Why does the ship have to fly away once you enter, and dock again when you want to exit. Can't it just stay docked, and you leave by the door as in the first game.
Truly, I think everyone who is considering playing ME2 must play ME1 first. Obviously this won't happen in a lot of cases, but at least I've said it.
It would be a shame if someone plays ME2, thinks it is cool and wants to find out about the first one, only to find it is a little less streamlined.
Again I want to emphasise, I am enjoying ME2 and it is a worthy sequel, full of humour and substance. Just don't go expecting ME1 to have a similar Halo/Gears feel as ME2. Especially the PC version.