If you can afford the 980 ..... your budget will certainly allow buying the best you can get. With these CAS 6 currently at CAS 7 prices, jump on it quick.
$190
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226121
I don't see you OC'ing past 200 BCLK w/o LN2 and DDR3-1600 is just fine for that:
Stock
133 BCLK x 25 CPU Multiplier = 3.33 Ghz
133 BCLK x 8 CPU Multiplier = 1067 MHz
20% OC
160 BCLK x 25 CPU Multiplier = 4.00 Ghz
160 BCLK x 10 CPU Multiplier = 1600 MHz
Seems like BCLK of 160 will get you way past where you wanna be and is stable. Past that you'd prolly have to cut CPU multiplier