You may want to try digital zoom if you do buy the GRD IV - it has an optional feature called auto resize which effectively performs an in camera crop and saves the result at the approprite number of mega pixels. If you read the comments on Pavel's test linked above, one way of avoiding the vignetting using the GT-1 is to crop out the corners effectively giving you a reduced MP picture...well you can do the same with digital zoom without all the expense and bother. I did some basic comparisons here between the GT-1 on a GRD and digital zoom on a GRD IV:
http://ricohforum.com/viewtopic.php?f=66&t=8550Before I tried this I belonged to the 'never use digital zoom' brigade but I now have the rocker switch set to digital zoom and use one press (more or less '35 mm') quite a lot and two press ('40 mm') a bit too and the results printed at A4 are fine.
Richard