what about raising ram mhz speed ?
It's generally accepted that power consumption of components is proportional to frequency. Therefore if you overlock by 10%, you'd add 10% to the power consumption. If a RAM stick is 2W then you'd add 0.2W. These are tiny amounts. Changing voltage of RAM can make more of a difference but it is still only going to be a couple of Watts per stick which as a proportion of the total systems consumption, is minimal.
how do i know for sure if i really need more ram?
i think that my everyday applications are not so demanding, but when i open some game like the sims 3, it takes a lot of time until it is loaded ( it's not a very demanding game but it has o lot of content), during playing the game it looks like o.k.
and while the game is running on background, even without playing, every simple act on the pc is very, very slow.
So it sounds like you might have used up the RAM and be working in virtual memory. This is slower as you've seen.
Before adding more RAM I would instead look at how your system uses RAM (under Windows use the task manager to see how memory is used) and save as much as possible:
1. Remove/disable all hardware not required including unused devices on the motherboard. The drivers for each device use RAM.
2. Go through the services started by Windows and disable those not required:http://www.jasonn.com/turning_off_unnec ... windows_xp
3. Look at applications that are running in the taskbar. Uninstall everything that you don't need. This is a common area for RAM to be wasted and it can add up to the hundreds of megabytes.
4. Diasable things in Windows that you don't need. Aero for example or Windows sounds for when you switch on/off your computer.
No point in wasting money on more RAM if you're already wasting RAM itself with things that you don't need.
Windows tends to be pretty bad at handling memory usage and it's something I notice as a Linux user whenever I use a Windows system. When gaming under Windows things run out of memory and then run slow whereas on Linux you tend to see it paging silently in the background preemptively so that you never get to the point of completely running out.