What if Hollywood did a modern re-telling of...

...Double Indemnity?

(disclaimer: I do not think this would be an advisable course of action. The classic noir can't really be improved upon, in my opinion. This is simply a hypothetical exercise and time-waster, nothing more.)

Re-Casting:

Walter Neff (originally played by Fred MacMurray): Jason Sudeikis

Phyllis Deitrichson (originally played by Barbara Stanwyck): Charlize Theron  

Barton Keyes (originally played by Edward G. Robinson): Tom Hanks

Mr. Dietrichson (originally played by Tom Powers): Harvey Keitel

Lola Dietrichson (originally played by Jean Heather): Mary Elizabeth Winstead

Nino Zachetti (originally played by Byron Barr): Richard Madden

My thinking in regards to re-casting...
Jason Sudeikis as Walter Neff may sound like an incredibly odd choice, but there are a few key reasons I'd like to see him in the role. Before Double Indemnity, MacMurray had taken comedic roles almost exclusively. I think that, because of Sudeikis' history as a comedic actor, no one would expect him to take on a serious role - or to do well at it. I think he could pull it off, though. And he looks remarkably similar to Fred MacMurray, which doesn't hurt. Choosing a Phyllis was difficult - it needed to be a talented actress, slightly older (late 30s, early 40s), beautiful but capable of looking "cheap". I think Theron is a good fit here - Monster proved she has acting chops, and I don't doubt she can play a convincing femme fatale. Choosing Tom Hanks as Keyes was mostly influenced by his excellent performance in Catch Me If You Can - also working in the capacity as an investigator of fraud.

Re-scripting:

If the story is set in modern day, there are obvious changes that would need to be made. I think the incorporation of new technologies into the story could be done effectively, but only if it is worked into the script in a natural, subtle way. For example, instead of meeting each other at a set place, Phyllis and Walter could contact each other using cheap, throwaway cell phones. When Walter is recording his confession, it could be via webcam instead of the dictaphone. Keyes could use GPS tracking, wire-tapping, and internet spying techniques to observe Neff and Dietrichson...of course, this would mean that Neff, being aware of these types of surveillance, would find work-arounds. And it seems more realistic that Keyes would head a team of fraud investigators, rather than working alone.
I think that the basic plot of the film should stay basically the same - femme fatale convinces an otherwise good guy to help her bump off her husband and collect the insurance money. The story itself has now become a cliche...I think that it could still work, if done in a meta-textual way. The characters should be aware of how crazy their scheme is, but convince themselves (as a result of hubris) that they are too smart and organized to be caught. I guess the closest analogy I can think of would be a modern day heist film - the idea of "pulling a heist" is a Hollywood cliche, yet it can still make for a compelling film.