A lot of companies sell fixed sites that fit the rear dovetail. Even if they don't list nightsights as an option, other companies will install tritium inserts for a small fee.
Chip McCormick makes a very interesting rear sight that completely covers the rear dovetail, but I'm not sure how it attaches to the gun. It's very smooth looking and I'm keen to see one in person.
XS Sight Systems, formerly Ashley Express, makes a top-shelf set of fast-acquisition sights designed for combat more than target/accuracy shooting. Everyone raves about them.
MMC is another company that will gladly help you find something that you might like.
The worst part is desiding on a style. The McCormick full-coverage sight is neat and clean, and certainly different, but the XS sights are more "professional" - they're built for fighting.
