Well your PSU is a 450 watt PSU. Your CPU is using 95W (non-OCd), RAM, Mobo and such should only pull about 50W I think. The GPU you have runs at about 50W and the 6870 runs at about 150W. I would think you would be ok but you might be cutting it close. Using Neweggs PSU calculator:http://images10.newegg.com/BizIntell/tool/psucalc/index.html
You should get right around 430W. eXtreme Power Supply Calculator lists the wattage at ~400W so you should be fine, but it could be close.