That would be a question that you yourself would have to address for determining the feasibility of such a task depends mainly on cost. Consider these points, if you are upgrading your computer because it has considerably slowed down, then a total retrofit or even replacement would be in order. This would be more akin to older machines that may have already been serving you for a good couple of years, besides, you may not be able to find any compatible hardware anymore due to the fast pace at which technology advances.
Should you decide to do an upgrade simply because you need more space of memory, then by all means do so, but do not expect radical increase in performance. Why, memory upgrades simply mean you are making more space available for use by your computer. This has no bearing on speed for you may be able to run more apps at the same time but the processor and the bus speed are still quite the same. Compare it to a car, you get some gas and run a distance, add more gas and you get more distance with the same engine.
There may be some available parts at specialty surplus stores that keep new yet older computer parts but they may come at a premium and their reliability can be questionable due to their age. Balance the need for speed and the need for performance that can be addressed by an upgrade or a total replacement, the later being the best bet for it assures you have enough oomph for more demanding applications that are coming out today.