agree, with above. Your motherboard will not support anything you throw at it. You need to know at least the type..
Spending money on RAM for expensive timings is not cost effective to get you real world performance gains so, just get whatever meets your specs.
Prices for RAM are pretty competitive for each spec, so among whatever is compatible, get the cheapest or within a few dollars or so of the cheapest.
It is a commodity like gas. For the most part gas is gas, go for lowest price (but just like gas there are minor variations for special purposes, but in general, it doesn't make a difference).