Where to upgrade system first?


Dec 26, 2010
I have an amd Athlon II x2 245 2.9ghz, 4 gigs of ram 1333(2x2), msi radeon 5450 1gig 64bit video card, asus motherboard m4a785td-m evo, 160 gig hard drive, 500w ps from tigerpro. my resolution is 1920 x 1080. I want to upgrade to windows 7, do hobby video editing and photos, and am a gamer for sc2. My thoughts were to first get a video card possibly asus radeon 6850 or 6870 budget on that would be around 160$. then I was thinking of going for a Phenom 4 955 be cpu for 120$. Is my 500 watt psu large enough for one card and possible for crossfire of those two cards. Then I was thinking of upgrading the psu to a 750 watt and get 2 1tb wd hdd and raid for performance. any ideas let me know. Should i be taking a different route like ram for example? or motherboard? Thank you!