This is a great article on how and why the Mass Effect movie could actually be good, as opposed to just about every single other video-game to movie adaptation. It also features the best casting list I've seen:
Matthew Fox as Commander Shepard
Zachary Levi as Kaiden Alenko
Ron Perlman as Urdnot Wrex (even if Wrex is just CGI, Perlman should be doing the voicework for him)
Emily Blunt as Ashley Williams
Mark Strong as Garrus Vakarian (Could just be CGI'ed and voiceover'ed, so Mark Stong isn't a must have choice)
Ellen Page as Tali Zorah nar Rayya (Same as above, I see no reason Ellen Page needs to play the role...could be anyone, really, since we never see Tali's face)
Liv Tyler as Liara T’Soni
Cate Blanchett as Matriarch Benezia
Hugo Weaving as Saren Artemis (Again, just a CGI'ed model would do, but Hugo Weaving would be a good choice for voice work for the character)
Helen Mirren as Doctor Chakwas
Seth Green as Joker
Keith David as Captain Anderson
Pete Postlethwaite as Ambassador Udina
Paul Bettany as Thane Krios (CGI would do, but Bettany would be another good choice for VO work for Thane)
David Hyde Pierce as Mordin Solus (Another CGI character, Pierce would fit Mordin's voice very closely)
Yvonne Strahowski as Miranda Lawson (A quick dye job and she's ready to go)
Harold Perinneau as Jacob Taylor
Noomi Rapace as Jack
Michael Clarke Duncan as Grunt
Tilda Swinton as Samara
Jason Isaacs as Zaeed Massani
Jennifer Garner as Kasumi Goto
Martin Sheen as The Illusive Man
Seriously, click the link above and check out the pictures of the characters in comparison to their casting choices. The looks match up almost perfectly, and in just about every case I'm sure the suggested actor is more than capable of pulling off their respective roles. Seriously, this is a great casting list. Maybe a bit unrealistic considering the salaries of most of those names, but still, were it to happen this way, I'd have not one complaint.