okay! i'm on the same road as of you.. cause i am building a gaming pc aswell with almost the same budget!
now if your gonna go for a mid level graphic card i would suggest you have a good nice cpu! atleast a sandy bridge one or a quad.
Now you only have 2 options in that price range:
1) Nvidia gtx 550 ti
2) AMD HD 6850
both of these cards are fairly old but they perform really really good in todays time.
which one performs better?
6850 has a slight edge over the gtx500 but it also consumes slightly more power which is not really a issue.
both of them support SLI and Crossfire! + Multi-monitor support.
and about the power supply! it doesnt really matter.. anything above 400 watts will keep you safe! but if i was you i would go with the 600w corsair or antec power supply (modular if possible). cause in case i feel like upgrading in future i can do that!
lemme know if you have any other questions! preferebly PM me
edit:
Also like others said Nvidia has better driver support! i mean they keep releasing new drivers very often, but i'll recommend you go with the AMD card cause there more powerful.