There is no question you installed many third party system modifications, any one of which, or the combination of all, could be responsible for your system performance.
One quick way to check if your User login or launch items are to blame would be to create a new, "temporary" user. Log out of your usual account and log in to the "temporary" one. If your system's behaviour improves, then the culprit(s) lie in the items you selected in System Preferences > Users & Groups, Login Items. Un-check one or more then test the results.
To do this read: Isolating an issue by using another user account
I am unfamiliar with many of your User login items and kernel extensions (GeekTool, ZangZing, PowerboxInjector, RelaunchFinder, Tao Effect, Netculator). For those with which I am familiar, the Western Digital MyBook driver has been implicated in similar problems, so unplug the device to observe any change. If this results in improvement search Western Digital's website for an update.
You can see for yourself Sophos has its tentacles all over your system and could also be a causal factor. Anything that modifies your system has the potential to affect its performance. The decision to use such utilities is yours but it must be accompanied with the awareness of how it works, what benefit it conveys, and the limitations inherent in its design. Given the fact that the threat of Mac malware is so minimal, you have to question its value. Sophos did nothing to prevent you from installing the execrable MacKeeper or many system hacks that can contribute to system instability, slowdowns, and crashes - nothing can do that except you.
Simply stating that you don't have enough RAM is incomplete. It is better stated your Mac may have inadequate resources to run acceptably, given all the modifications with which you have burdened it.
For example, here is the applicable EtreCheck excerpt for my MacBook Air, which also has 4 GB RAM:
Kernel Extensions:
at.obdev.nke.LittleSnitch
Problem System Launch Daemons:
Problem System Launch Agents:
[failed] com.apple.afpstat.plist
[failed] com.apple.lookupd.plist
[failed] com.apple.mrt.uiagent.plist
Launch Daemons:
[loaded] at.obdev.littlesnitchd.plist
[loaded] com.barebones.authd.plist
[loaded] com.bombich.ccc.plist
Launch Agents:
[loaded] at.obdev.LittleSnitchUIAgent.plist
User Launch Agents:
[loaded] com.ecamm.printopia.plist
User Login Items:
iTunesHelper
SpeechSynthesisServer
3rd Party Preference Panes:
Flip4Mac WMV
Perian
Printopia
Internet Plug-ins:
iPhotoPhotocast.plugin
JavaAppletPlugin.plugin
QuickTime Plugin.plugin
Silverlight.plugin
User Internet Plug-ins:
Bad Fonts:
None
Top Processes:
1.7 % WindowServer
0.8 % EtreCheck
0.6 % System
0.6 % activitymonitord
0.4 % hidd
0.4 % Safari
0.1 % fontd
0.1 % coreservicesd
0.0 % mds
0.0 % Activity
The above system runs fast with no objectionable slowdowns, even after loading nearly every RAM-intensive program I have installed: Safari, iMovie, iTunes, iPhoto... many others too numerous to mention. I have tried and failed to find limitations that would make it run unacceptably slow, but I use almost no third party system hacks.
More RAM is always better but adding more would be masking a symptom instead of fixing the problem. The problem is not Mountain Lion. Fix the problem.
