I would suggest going for a 3rd gen i5 / i7 those will be the best for gameserver hosting.
Should fit fine in your budget with SATA3, if you can shell a bit more i would suggest getting SSD Drives for performance.
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[FONT=Arial][COLOR=#333333]Intel® Core™ i7-4770
Processor (8M Cache, up to 3.90 GHz)[/COLOR][/FONT]
[FONT=Arial][COLOR=#333333]16 GB 1333MHz[/COLOR][/FONT]
[FONT=Arial][COLOR=#333333]1TB SATA 6Gb/s NCQ 32mb Cache Barracuda[/COLOR][/FONT]