Any rumours on the sensor Mp size for this next body in the popular APS-C range - and when it is due out?
The 450D had 12.2 Mp sensor
500D had 15.1 Mp
550D/600D have 18.0 Mp
650D - 22 or 24 Mp?
If you are shooting smallish targets requiring cropping then increasing pixel density on the same sensor must be beneficial to image quality - same size image but more pixels covering it?
Well, there are issues of spatial resolution to take into account. A 24mp sensor is going to be pushing 130lp/mm. Modern camera lenses can only achieve such resolutions at middle apertures. At f/5.6, you only have 123lp/mm due to the effects of diffraction. Anything smaller than f/5.6 will produce increasingly less spatial resolution due to greater and greater effects of diffraction. At around f/4...perhaps as low as f/3.5 or so, in most cases for professional quality glass, optical aberrations and diffraction normalize, and you reach your maximum spatial resolution. I find f/4 to be a safe bet for maximum spatial resolution, which would be 173lp/mm. At larger apertures than that, optical aberrations will quickly dominate, and affect spatial resolution more than diffraction does at f/5.6 and on, often reducing spatial resolution to as low as 30-40lp/mm wide open (depending on the lens.)
There are a very few lenses that achieve near-perfect resolution at very wide apertures, but they are less common than the average DSLR lens. Zeiss has a lens or two capable of around 400lp/mm at around what I suspect is probably f/1.5. Some of Canon and Nikon's top-end supertelephoto lenses are probably capable of nearly 173lp/mm at their maximum apertures of f/4, and for top-end telephoto lenses like 300 and 400 f/2.8's, your probably capable of a couple hundred lp/mm. Those are all extremely expensive lenses (i.e. ten thousand dollars give or take a couple thousand) that few people who are going to be using either a D3200 or 650D entry level DSLR would be using.
There are also the issues of total system spatial resolution, which is effectively a mean of the spatial resolution of each component in the system. In this case, if the sensor is capable of 120-130lp/mm, and the lens is capable of 173lp/mm at f/4, your actual total system resolution is going to be a lot lower. You can certainly keep gaining improvement by continued increase in pixel density, but your going to encounter diminishing returns. The more you push sensor spatial resolution towards 170lp/mm, the narrower and narrower the aperture range is going to be where you can actually maximize your sensors potential. You might also run into other consequences...such as images that look fairly soft @ 1:1 crop at apertures outside of that narrow band of maximum system spatial resolution (this is part of the problem the 7D with its 18mp sensor has...its 116lp/mm spatial resolution is only viable at a relatively narrow band of apertures around maybe f/3.5 to f/6.
I think Canon may be at its limits with spatial resolution until it can make some of the same sensor improvements Sony has made to their Exmor sensors. Noise is a bit of a problem at ISO 100 and 200, and SNR is a bit of a problem at higher ISO's. At the very least they will need to migrate the 1D X and 5D III sensor improvements into their APS-C manufacturing. They might also gain from the use of backlit sensor technology as well. Nikon is probably in a better position to make a 24mp APS-C sensor produce better images that don't look as soft because of the very low read noise in Sony sensors...but there is still the question of whether it will actually improve things all that much for someone who is looking for an entry-level DSLR and is less likely to be using professional glass. Entry-level glass is unlikely to achieve maximum optical spatial resolution at any aperture, diminishing the value of having a sensor capable of 125-130lp/mm.
There is limited room to grow sensor resolution in APS-C formats, and far more room to grow spatial resolution in FF formats. Outside of also producing new entry-level lenses that approach perfection at wider apertures, with significantly reduced optical aberrations as wide as f/2.8, higher pixel density won't offer nearly as much benefit as better glass.