This is what I would get
case-NZXT Crafted Series Tempest 410- $60
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146081
Motherboard-ASRock FM2A75 Pro4-M FM2 AMD A75-$75
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157334
PSU-SeaSonic G Series SSR-550RM 550W- $90 (on sale)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151119
Cpu-AMD A10-5800K Trinity 3.8GHz -$130
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113280
Memory-CORSAIR XMS3 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600-$70
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233202
HHD- Western Digital WD Blue WD10EZEX 1TB 7200 RPM 64MB-$75
http://www.newegg.com/Product/Product.aspx?Item=N82E16822236339
Dvd-rw/ cd-rw drive-ASUS 24X DVD Burner-$20
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135204
Grand total about $520 then whatever shipping charges are.
I think the Seasonic PSU here will serve you very well.Trust me you will love the modular cables. My Seasonic X-850 850W serves me well.
Also you can add 4 more case fans to the NZXT 410 tempest later. I would recommend Cougar fans or Noctua fans (get these ones from NCIX they are cheaper there). And if you wanted to later (since you you would have 550W of power) you could add a 660 Nvidia card or AMD 7850 or AMD 7870 down the road if you needed more performance.
About anti-viris software. Microsoft has Microsoft security essentials and its free if you own a Microsoft OS.
Civilization 5 system requirements
Recommended Requirements
Operating System: Windows® Vista SP2/ Windows® 7
Processor: 1.8 GHz Quad Core CPU
Memory: 4 GB RAM
Hard Disk Space: 8 GB Free
DVD-ROM Drive: Required for disc-based installation
Video: 512 MB ATI 4800 series or better, 512 MB nVidia 9800 series or better
Sound: DirectX 9.0c-compatible sound card
DirectX®: DirectX® version 11
So this should play it pretty well. If not, just add a graphics card then you should be good.