How much RAM (Random Access Memory) do you have in your machine now? How much RAM will that machine accept if you do decide to upgrade it along with putting in an SSD? I think both several of the sites (such as Crucial) which sell RAM will provide charts which show you exactly which RAM chip(s) and which capacity are most suited for your machine.
As for the SSD, that will provide a lot more speed than your current HDD, and again you will need to take capacity into consideration, especially if your current HDD is fairly large. Prices on SSDs are still pretty high and so getting one with a decent amount of capacity (say, 512 GB) could be fairly expensive, and definitely getting one which is a TB or more would definitely fall into the realm of expensive!
So: two things to consider, here. One is how much RAM your machine will be able to handle and the second is how much storage capacity you want in an SSD and how much you can afford. In addition, many people go for buying an extra external HDD or external SSD in order to have the needed extra storage space while not putting all of their money into the internal SSD.
In terms of how much storage is realistic, well of course that depends upon what you currently do with the machine and what you already have stored on it. Large iTunes library? A bunch of video files, both commercial movies and user-rendered and created videos? Maybe a whole lot of documents and PDFs or perhaps you've got a pretty large image file of photos..... Assessing what you already have and how much space you are using plus taking into consideration room for growth will help in evaluating and deciding what storage capacity you will need and want in that SSD. As for RAM, again a lot depends on what you do in the machine, and if you do extensive work in photography or video, or generating lots of documents and PDFs and other things, you will need more RAM than if you simply use the computer for emails, forum browsing and participation, listening to music, watching movies, etc.
As for attempting to formally "monitor" your memory (RAM), that's something which i don't pay much attention to. If I've been doing a lot of photo editing and image retouching as well as other things and I notice that the machine seems to be slowing down, not handling the functions as quickly, then a simple reboot takes care of that. Ditto if I've been watching a movie or two and also doing some tinkering in iTunes; at the point where I see that iTunes is balking a bit, again a reboot refreshes the RAM and we're back in business.
Macs are not Windows machines; they don't need a lot of "monitoring" and extra third-party software, etc., to ensure that they are running smoothly. They do quite nicely on their own without all that and in fact some third-party software which is supposed to be helpful actually is more of a hindrance than a help and beyond that, some can actually be more harmful than helpful. We've had this discussion before, actually.....