If you have a low budget, a dual core AMD system with cheap/onboard GFX should do fine. It won't max it (a better vid needed for that), but it will play it fine.
Case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811233062
$22 GIGABYTE gz-ph2a3 (cheap case)
PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371034
$44 Antec EarthWatts Green EA-430D (good low price PSU)
MB:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138179
$50 BIOSTAR A770E3 AM3 AMD 770 ATX AMD (cheap ATX AM3 board)
CPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103903
$60 AMD Athlon II X2 250 Regor 3.0GHz (retail, so has a HSF)
RAM:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231423
$25 G.SKILL Value Series 4GB 240-Pin DDR3 SDRAM DDR3 1333 (if you get a 64bit OS, get a second one of these later)
HDD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148699
$37 Seagate Barracuda ST3250312AS 250GB (cheap SATA HDD)
DVD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106289
$19 LITE-ON DVD Burner (SATA)
VID:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125378
$47 GIGABYTE GV-R645D3-512I Radeon HD 6450 (cheap video, can spend more on a better one)
Total, no OS: $304 + S&H (not counting MIR, ro special/combo deals)