don't think any number has been confirmed yet, but for coop play i doubt we'll see more than 4, possibly 6 at the very highest. I think 4 or 5 is more likely. 4 being the standard from the past, and 5 allowing for 1 of each class to possibly play together
4 might actually be the maximum. You need to take into account that every effect is pretty big and if you have a lot of people using a lot of big moves, there might be a little lag. Blizzard has said that they want the game to run seamlessely, and they don't want to upset the flow of action with lag, so by 2011 standards, video cards might be able to handle 6 on screen.
Take into account native resolution and all that other crap, 4 might be a very likely number for co-op.
Some of the lag would be from the video cards but most of it will be comming from the servers with that many games open with that many people in them doing that much action
The question of 4 vs 8 has less to do with netcode and graphics cards, and more to do with D3 being primarily geared for solo and small team play. All of the classes are self-sufficient, there are no support characters or intention to promote a balance of classes in a party. Furthermore they all have over-the-top effects and powers that could make play too chaotic in large numbers, not to mention the problems with scaling monster difficulty appropriately. Personally I found 8-player D2 parties over the top and not enjoyable, and opinions will vary, but it seems the D3 designers have some similar concerns.