AV software is not needed if you employ the proper
security steps on your Mac.
Currently there are
zero viruses affecting Mac OS X in public circulation, but there are other kinds of malware existing, that can infect your Mac.
To learn more about malware in Mac OS X and what steps can be taken to protect yourself, read the following F.A.Q.:
As for knowing when to get new RAM, open
Activity Monitor and go to the
System Memory* tab and look for
Page Outs and
Swap used after a day of typical use and if the Page Outs are more than 750 MB you can profit from additional RAM.
As for the HDD, it depends on how much storage or how fast a storage medium should be for you, there is no general software to tell you when to get a new HDD, unless you look at its S.M.A.R.T. status and it shows as failing. That is what a backup is there for.
As for logic board (mother board), you cannot replace those that easily with a any Mac.
If you want to enhance the performance of your Mac, be sure to check these two articles, do not just use applications, that promise to do it for you.
Articles explaining why Mac OS X does not need maintenance software like CCleaner, MacKeeper or CleanMyMac: