Will a 1gig 4890 crossfire with a 512mb 4870? I thought the memory size needed to be the same between the two cards or will it just recognize the 512mb from the 2nd 4870? It may be a better idea to just go with a 6870 for ~ $150 and sell your 4890.
4890 - $70
4850 - $50
Total - $120
For only $30 more you can go with a solution that's almost as fast as the crossfired cards and you'll more than make up the difference in a year of solid use when on your power bill since the 6870 uses significantly less juice than your single 4890.
Not to mention prices on the 6000 series AMD cards should be dropping in around two months.