How can I constantly monitor my computer's RAM usage?
You can use any number of dock/bar/gadget/widget type of tools and programs. Some stay on top, some cut out their own chunk of the screen, some are transparent, some are pegged to the desktop, most are configurable.
WinBar has a module to display the RAM usage (I personally like this and use the 1.2.95 version)
The Windows gadgetbar/sidebar has built-in (CPU Meter) and third-party gadgets that can display the RAM usage.
Samurize has a RAM widget
RocketDock has a RAM gadget
StarDock’s Object Desktop / Desktop X have RAM widgets
Konfabulator / Yahoo! Widgets have them too
Google Desktop may be discontinued, but it is still available and can do it
There are multiple software written for such a solution. Just a few after a quick google search:
Taskbar Meters (Download)
RAM CPU Taskbar for Windows 7 (Download)
And one that is quite light weight (but doesn't support any Windows version newer than XP):
RAMpage
Either way there are scores of these programmed for all kinds of specifics. So go ahead and choose any one of them. Also I don't think it would be a bad idea to go ahead and upgrade your RAM, 4GB is quite little.
You wrote :
something is going to crash because I'm using too much RAM
What you mean by "crashing"? Normally if there's too much RAM usage the system start to "swap" from the RAM to the Pagefile.sys and not "crashing"... This swapping may slow down the performances but nothing else.
1) The Virtual Memory: RAM + Pagefile.sys can be increased by adding RAM or increasing the pagefile.sys...
2) The unused Memory is a lost Memory If you run many applications at the time they can't run faster when there's more unused memory...
3) The main bottleneck in performances don't comes from the lack of Memory but from the percentage of CPU / GPU usage. What you have to check is the peak usage of memory, the % of actual usage and the total CPU time.
BTW: check the CPU usage in your screen capture: 93% !
Now to check in "real time" the Memory usage I suggest you to use MS TechNet Sysinternals Process Explorer and set the columns to see the total CPU usage, the total GPU usage and the CPU time. Set the systray icons to see the graphics of the CPU history, GPU history and Physical Memory history as you want.