I would update the Motherboard and RAM to DDR3 and the Graphics cards.
I'm not sure what your budget is or the wattage or performance of the 4870 in Tri XFire (Guessing 6-700+W for the Graphics alone), but getting this should be safe for starters:
http://pcpartpicker.com/p/VmLPnQ
http://pcpartpicker....nQ/by_merchant/
Gigabyte 990FXA-UD3 Motherboard
8GB (2x4GB) of G.Skill Ripsaw X Series DDR3-1866 CAS 9 RAM
Sapphire 7970GHz Edition 3GB GDDR5
Total Cost: $691.75 US
The 7970 Blows the 4870 Out of the water and due to lack of official SLI/XFire Support for MWO, you would be limited to 1 Card Anyway, and I think your CPU should still be decent, more so if you can OC it.