TO ADD A CARD OR TO REPLACE A CARD?

So i'm thinking of upgrading my motherboard and CPU, i currently have a MSI P55A-GD65 with a Core i7-870 Lynnfield 2.93GHz and i'm thinking of going with a Core I5-3450 and Gigabyte GA-Z77X-UP4 TH within the next week.

I allready have a Asus Rog Matrix 5870, so do i add another card so i have two 5870 for 200 bucks or do i go with one new XFX FX-787A-CDFC for 250 bucks?

or do i even upgrade the board and cpu and just keep them for another year and upgrade the video card or add another one so i'm running two 5870's

What do you guys think?

Thanks for the input