MechaHerc, you'll get a black circle around the corners. If it is a Sony DT lens, the A900 automatically activates APS-C mode, so you will never get the black circle.
Of course, best understood with a hands-on.
achew, the exposure is the same for a F1.4 on FF or APS-C. The angle of view will be different.
The depth of field will be different also because of the size of the sensor. On FF it is more shallow.
For example, to frame a head + shoulder shot using a 50mm lens on FF, you might need to stand about 1 meter away. On APS-C though, the angle is narrower, so you need to stand further away to fit the same subject e.g. 1.5 meters away instead. And as you know from your Tamron 90mm F2.8 Macro, the closer you focus, the shallower the DOF... that happens in the case of the FF also being nearer to the subject.
Of course, since the distance is different, the perspective is different also! Therefore to keep the same perspective you need to stand 1 meter away on APS-C. But it's not wide enough. So a 35mm lens will give almost the same angle of view as the 50mm on FF, while letting you be the same distance from the subject.
I can't remember if you have the A300/A350 but you can see what crop does if you enable 2x Smart Teleconverter; you can see the DOF become much deeper because you need to stand further away.
Millenia3000, I would rate the F58 higher on priority really.