I have an Early 2009 MacBook -- last of the white plastic ones. Lovely machine. Runs Mountain Lion.
I've put an SSD in it, which massively improved it.
I've just added a 4Gb RAM module, alongside a 2Gb stick, to take it up to 6Gb. The extra is worth any possible speed loss from not having pairs (but I don't think it's noticeable**).
It was one of the last MacBooks with FireWire, which I still find useful.
I use it every day, and take it to libraries to do my PhD research.
I don't run any monitoring software, which means I have no problems with temperatures, fan speeds, network activity, disk activity, memory management, processes, yada, yada, yada.....
** People have been talking about paired memory since the early 90s, but I've never had any problems with asymmetrical banks on any of my Macs, or ones I've worked on.