Here is a Crossfire configure chart from AMDhttp://game.amd.com/us-en/content/images/crossfirex/CF_combo_chart.jpg
You can crossfire any brand, If you have a EVGA and a ASUS HD5850 they will work together fine
If you have a HD5850 and a HD5870 they will work also.
A HD5850 and 1080i will run most games great. If you can swing it get a SSD. That will make boot time and load times a lot faster. The can you get another HD5850 later if you want.