It depends on how you use your machine and how much ram you currently have.
If you're doing video or a heavy Photoshop user and only have 512mb of memory yeah, your going to see a big difference. If you are a light photoshop user, or mostly business apps and have a gig of ram, well then you may not see much of a speed bump by adding 2 gig of ram.
The video card, and hard drive speeds are two other components you focus on to increase your speed.
As for OCW, I cannot speak to the quality as I never bought memory from them. I've been using crucial ram, a little more pricey then some others but I've never been disappointed and their service has been great. I've seen too many threads here and in the PB forum of people complaining of bad ram. I'm a big proponent of "you get what you pay for" I paid a lot for my G5, no use in cutting corners and saving a few bucks now, only to haunt me in the future.
Mike