If you want to be able to use dreamweaver to edit things then read this one
http://forum.x-cart.com/viewtopic.php?t=74
then while your at it you may find these interesting if your new to it all
http://forum.x-cart.com/viewtopic.php?t=876
Now that your in a reading mood why not give this one a go
http://forum.x-cart.com/viewtopic.php?t=6489
There you go, you should be well on your way now ....
I can tell you though that when V3.5.0 comes out in the next few weeks (ish) then one of the new features will be that all email will have a plain text and html versions.