For modern SLRs, ISO1600 is not that much of a problem. A lot has happened since 2006 - cameras, such as all technology, evolve.

Now I also think sales advice per se is not generally well liked here, so I will try to be general on the topic of upgrading:

The problem: An old camera no longer suits the user's need, what should be focussed on when upgrading.

1. Does the user require old lenses to be compatible? If yes, only the newest models with APS-C sensors will be a viable option if the user had an APS-C camera.

2. The user is able to upgrade all equipment and so may buy any SLR including lenses.

If 2) applies, there are other points to focus on:

• A full frame sensor will give a shallower depth of field - some people have issues with it, but it allows the medium of photography to be used to its fullest extent. How important is depth of field control to the user?

• One loses "reach" with a full frame sensor as it offer a wider field of view on an identical focal length. This can be an issue if the user carries forward lenses. For example, a 50mm lens on an APS-C sensor camera will give the same field of view as an 80mm lens on a full frame camera. (Canon APS-C with 1.6 crop)

• In general full frame will offer better higher ISO performance in the same sensor generation due to larger pixels. However over different sensor generations (!!) it may no longer hold an advantage and total image noise should also be compared if resolutions are significantly different. (What looks like more per pixel noise might look very similar in the context of the full image.)

The last point worth considering: Lenses "age" as well. Professional lenses are getting better, but the improvements are most significant with entry level lenses. Better lenses can be manufactured at ever lower prices. An old lens might not have the resolving power to fully benefit from a newer higher density sensor and might be best replaced too. A newer camera may produce disappointing results if the limiting factor is the lens.