yeah buy a 4gb stick for it it should take 5 min max and is well worth it for about 20 bucks. also if you wanna get a bit more adventurous by a ssd for it, something like a 128 or 256gb if you need the space and this will make a massive difference will feel as if it cost 3 times more with the loading time reduced heavily!
I see, hopefully it does work, do you think if I upgrade it until 8gb ram will makes much difference than to 4gb ? Also, if i buy the SSD, does it mean that I have to replace my old hard drive inside the laptop, or I can use it in addition to the old one ?
You likely have a 32 bit OS, if so, more than 4 GB isn't going to help (after 3.5GB, the rest gets ignored). That'll be something to check. An SSD upgrade as Jason suggests will make a difference (even in battery use). I'm going to suggest heat as a probable issue which can be resolved by giving the laptop a very thorough cleaning - you should be able to check your temps using CoreTemp or SpeedFan