Ever since I started out with photography, I've been using an old, used Sony Alpha 200 DSLR. I was mostly okay with its mediocre performance, but lately my demands have increased, and I would like better low light performance and better resolution. Since I was happy with Sony and already have Minolta/Sony-A-mount compatible lenses, I looked for the best compatible Camera within my budget, and the Alpha 77 seems to fit perfectly. Also, in numerous tests, this camera was praised, despite being SLT. However, my photography friends, possibly fanboys, keep trying to convince me to get a Canon or Nikon respectively, even though in my particular budget there seems to be no obvious alternative to the 77 considering that I would need to get new lenses, too. One of their Arguments were that my old Lenses, one of them being from an old Minolta, wouldn't be able to 'keep up' with the A77's resolution of 24MP. Is that true?
This question is very similar to another one, although the latter deals with new lenses on old bodies.
First of don't listen to your friends telling you that Canon or Nikon is the way to go. All of the DSLR/MIRC manufacturers today make cameras that are capable of producing high quality photos and the limitation is the photographer unless you're extremely good. A person that is so skillful that he/she is limited by a certain camera brand knows this already and doesn't have to ask this question.
There is some truth in the argument regarding older lenses not being able to keep up with newer high resolution digital bodies. Older lenses were designed for use with film in an era where people weren't pixel peeping so older lenses are often not as sharp and have increased amounts of chromatic aberration when compared to lenses designed for digital cameras. Also digital sensors are more reflective than film, hence the coatings on the rear element of modern lenses to prevent these reflections lowering the contrast of the image, something that older lenses lack.
These aspects are not the only ones to consider though. Even new lenses might have trouble keeping up with the high resolution of the Sony Alpha SLT-A77 unless they are really sharp. Also, properties such as maximum aperture, quality of the bokeh and build quality may not have changed much. If you already own some expensive lenses, albeit old ones, you have to consider the cost of getting the same lenses for another mount. The lenses might be able to be used with adapters on another system but at a possibly lowered functionality.
The older lenses are likely not keeping up with the extreme resolution of the Sony Alpha SLT-A77. There are other aspects to consider though and even newer lenses might have trouble keeping up with this resolution.
The lenses that can keep up with 24mp resolution (if you're a pixel-peeping test-chart addict), no matter the mount system, all tend to be $1000+ lenses, so if you weren't going to be spending that much anyhow, which system you're shooting probably makes no difference, and most of us tend to stay in the system where we already have glass, given the expense and PITAness of swapping systems. And the Sony/Alpha system has autofocusing Zeiss glass in it, which is certainly capable of "keeping up" with a high-resolution sensor. The ZAs more than hold their own against the pro-level Nikkors or Canon Ls.
The only real reasons to move to Canon/Nikon would be if your new demands also included wanting a variety of choice in TTL-capable radio triggers for off-camera flash, or a lot of good native supertelephoto and exotic lenses (like, say, tilt-shifts). In other words, the things you can't find for the Sony Alpha system at this time. If the system as it stands meets your needs, no reason not to stay there.