Being that most PC's come with at least 4Gb of RAM now a days, it really doesn't make a difference as to how many icons you have on your desktop. Back in the windows 95 days, yes. Haha
On a phone though, having your normal shortcuts to whatever apps you use often pasted to one of your homescreens doesn't use up any memory. Now something such as a live wallpaper will because obviously it's running constantly. Widgets such as a weather, will use extra memory because they have to update. And if they are animated then they will constantly use more memory because they are always running just like the live wallpaper. Plus aside from the extra memory usage, they are using more battery as well. Live wallpapers to me are pointless. They do nothing for me but they take away battery and memory. Widgets on the other hand I can deal with depending on what they are. I only have the Gsearch widget on my phone as I can just open up whatever app I need instead of putting a battery hogging widget on my homescreen.
There's an app called Android Assistant that has many features such as cache cleaning, battery saving, battery stats, system stats, startup manager, app backup, etc... and allows you to set the app to automatically kill running apps to a certain time frame of your choosing like every 10 minutes, every 30 minutes, hour, etc..... I've been using this app every since I've had a smartphone and that feature is one of the main features I really enjoy. No need to kill apps when you lock your phone because the app will do it for you. Check it out if you haven't already, it's a great little app with a lot of tools.