Beyond about 8 MB, most photographers don't need or use the additional pixels. There are exceptions, but these people know how to use those pixels. The rest of us...
The flip side of megapixels is that the smaller the pixel, the more variation (noise) there is from pixel to pixel. The high mega-pixel point and shoot cameras have very high noise levels, which are masked by the built-in noise reduction, which then wipes out the detail... so you don't get the benefit of all those pixels. The cameras with big sensors and relatively low pixel counts have big fat pixels that need minimal noise handling - they give very even performance from pixel to pixel and allow superior picture quality.
I've used the XSi and my advice would be to consider upgrading lenses if you really want to spend money, and buying accessories such as tripod and flash. Those will give you far more improvement in picture-taking ability than "upgrading" to a larger pixel-count sensor.