Don't. You are already running W7 so its time to move to a DX11 video card. Don't do crossfire. ATI have been putting out horrible X-fire drivers lately. A lot of people have actually been losing performance with the 10.6 and 10.7 driver packages.
Either the HD5850 or GTX 470 would be around the performance of crossfired HD4850s and cost about $300 while giving you DX11 and using much less power, especially at idle. Factor in some money from ebaying your current card and it's the obvious way to go.
Either will be fine on your current PSU. Some more ram is a good idea, as is overclocking that processor.
+1 with Bluescreendeath. if you're buying a new mobo may as well change your cpu while you're at it, seems like a hassle to me without much benefit. unless you REALLY want to spend some money, just sell your current card and get a gtx400 or hd5000 series card