Good luck finding a card that supports hybrid crossfire, because IGPs are so weak compared to a decent discrete card only a few of the very low end units supported it. According to the crossfire compatibility chart from AMD it looks like the 3470 was the strongest unit to support hybrid crossfire, so its not really worth the effort when you can grab a significantly stronger 5570 for not too much.
It depends on how much you have to spend. For $120 or below there are reasonable choices from either side. For $150 the HD5770 is the way to go. For $200-250 the GTX 460 is the best card for the money. At $300 either the HD5850 or GTX 470 are good choices.
In terms of multi-card setups SLI tends to scale better than crossfire.
Look at the crossfire chart above, crossfire is quite a bit more lienent than SLI, you can crossfire a 5970 with a 5830 if you wanted or a 5770 with a 5750, it wont scale well if you do that but you can do that. They do not need to be the same brand or model number, as long as they are in the same family(eg 58xx/59xx or 57xx) it will work.