DDR 1333 8 GB 7 CAS vs DDR 1600 4 GB 7 CAS

The DDR3 1600 would be better but CAS 7 is probably very expensive compared to the much more common CAS 9 and you won't see any considerable/noticeable performance increase. If this is for a gaming machine you should know that no game see's any performance increase from using faster RAM.

Actually no, the price difference is very similar.

http://pcpartpicker.com/ca/part/gskill-memory-f310666cl...

http://pcpartpicker.com/ca/part/gskill-memory-f312800cl...

Which one should I purchase?

The pricing is not similar. The 1333MHz kit has double the capacity of the 1600MHz kit.
Capacity is much more important than frequency and latency. The 1333MHz kit has a decent price but the 1600MHz kit is very expensive for it's capacity and I recommend against buying the 1600MHz kit in this situation. Are these the only two kits you're willing to consider?

Right now I am looking for 6GB/8GB memory stick with 7CAS or less. It could be in the 1333 series or 1600 as the difference is very small for its price.
Also, my budget would be around 50$ for the memory sticks.

Thanks.

I am going to purchase these ones then!

http://www.newegg.ca/Product/Product.aspx?Item=N82E1682...(Desktop+Memory)-_-G.SKILL-_-20231445&AID=10592396&PID=3938566

One last question, how does this rig look? Is the motherboard good with this setup?

CPU: Intel Core i5-2500K 3.3GHz Quad-Core Processor
Motherboard: MSI P67A-G43 (B3) ATX LGA1155 Motherboard
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory
Video Card: MSI Radeon HD 6870 1GB Video Card
Case: Xion XON-570 ATX Mid Tower Case
Power Supply: Rosewill 700W ATX12V / EPS12V Power Supply

Rosewill psus are generally bad but other than that everything else is fine.

Of course the lower the cas and faster the speed is better if you can find them at similar price.

Best answer selected by aznpiggy.