...only significantly faster (and significantly more expensive) memory can deliver a tiny performance advantage.So if you're looking to extend the life of you sluggish PC, try some additional RAM -- but don't waste your money on the more expensive stuff.
Quicker timings (low CL values) are favorable, but you shouldn’t fork out considerably more money, as the differences are small.
On a related note, if you are looking for ways to increase your productivity, consider a 2nd monitor. You will likely need a second video card or one that supports 2 monitors, but the cost-benefit is well worth it. If you are lucky enough to have a laptop docking station that supports digital and analog video, you already have what it takes to have 2 monitors.