I moved from the A300 to the A550, which is almost the same size and weight as the A580, since the A580 is a follow up body in the same line. The A5xx series is about the same overall size as the A300 in your hand - though it is a slight bit lighter. The button layout to me is much improved, and more controls as well, which is a nice addition. The viewfinder is slightly larger too, with a slightly brighter view to my eye and I love the active grid matrix in the viewfinder. The live view system is also quite superior, in modes, LCD resolution, and tiltability of the screen. Of course, the camera is much faster to focus, as well as much better burst capability, and the buffer is nice and large too. High ISO ability is out of this world better - huge improvements from the A300 to A550, and the A580 is even a little better than that. Battery life is extraordinary, and again the A580 even slightly betters the A550, which is already impressive.
As for waiting on the A77 or going with the A580 - tough call. I guess it depends on how much you feel you need the upgrade - if you can wait it out, and see what the A77 has to bring, then I'd probably do so. I personally have not been enamored of the EVF cameras, or EVFs in general, so based on how they sit today, I'd go with the A580 every time. But the A77 is a future camera - we don't know if it will be the same, better, or worse than the A33/55, or if the EVF technology has evolved enough to be a legitimate replacement for an OVF without compromising key areas the current EVFs still do. I'm holding for the moment myself in upgrading my A550 - I hold judgment on the A77 until I've seen and tested it. If I feel they've addressed all the EVF shortcomings and issues, I may go that route next - if not, I'll hunt down a bargain A580.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses