3 x whatever GB for 1366 boards
2 x whatever GB for 1155/1156 boards
As for what I'd buy, I can only speak for myself. I started using GSkill memory when they were cheaper than the established brands. Over time and after some disappointments, the prices were the same so I stopped buying them.
You have left ou the most important info, that being what you are doing .... CAS latency impacts will change depending on what you are doing. For my needs, I always buy the lowest CAS I can afford.
1366 based systems
At 6GB (3 x 2GB), the best performance buy is the Mushkin Radioactives. The cheapest set I'd buy would be these CAS 9's from Corsair at $70
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145258
But when ya can get CAS 6 from Mushkin for an extra $20, it's a no brainer
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226181
At 12GB ya have to decide between 2 sets of the above or one set of 3 x 4GB. The low cost CAS 9 option is $155. Given CAS 8 is the only step up, I'd get two sets of the Radioactive CAS 6 modules for only $25 more.
1155/1156 based systems
At 4GB (2 x 2GB), the best performance buy again is the Mushkin Radioactives. The cheapest set I'd buy would be these CAS 9's from Corsair at $50 ($40 after $10 MIR)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145260
But when ya can get CAS 6 from Mushkin for $78, to me, it's a no brainer
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226180
At 8GB ya again have to decide between 2 sets of the above or one set of 2 x 4GB. The low cost CAS 9 option $ @ 3 x 4GB is $100.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145324
Ya can step up to CAS 7 for $155
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226178
For $1 more you can get two sets of the 2 x 2Gb Radiocatives at CAS 6 making this choice kind of a toss up.