What is your full make and model of your motherboard so we can see what is compatible with current BIOS.
As for your GPUs, it will be hard to find 4890 & 4870 with them being obsolete so you be better off getting a newer 5850 or wait for the new 6000 series cards to be released.
Now, your PSU won't handle a dual card setup of any newer GPU's or the older 4870 & 4890. You want at least a 750W from a reliable maker such as Corsair, SeaSonic, Antec or XFX. Tagan aren't the best... My 900W died with in a few months (but doing okay as a refurb though).