I take pictures on mostly small items such as jewelry for my clients, need to get some close-up details, so in this case I assume it doesn't make a difference which camera I use (T4i, T3, or T2i)? is the 60mm macro lens the right one for my purposes?
or does it even matter if I use a DSLR camera or not? would a good point and shoot still give me the same result if there is good lighting?
I'm not sure why image quality would be vastly different, except for maybe sample variation. They all use the same sensor (not sure if there were minor tweaks to the later generation).
The Autofocus on the T4i should be much better than the T3i/T2i, particularly when not using the center point. T4i uses all cross-point type AF points (even the Full Frame 6D doesn't have this). If you're going to be focusing using anything other than the center point, get the T4i. Regardless how good/bad a lens/sensor is, an out of focus image will always look bad
The Macro lens recommendation depends on if you're going to stay with a crop sensor SLR, or are planning on going Full Frame. If you're planning on staying on crop, get the 60mm EF-S Macro, wonderful lens and very sharp. Otherwise the 100mm Canon EF Macro can still be had for slightly over $500 I believe, and is also considered to be one of Canon's very sharp lenses. If you have money to burn, get the 100mm f2.8L Macro as it has better build, but more importantly an Image Stabilizer. Best bang for the buck would be the 100mm non-L Macro though. If Canon pares back their APS-C offerings in the future as rumored then resale of EF-S lenses may drop significantly.