i installed win 7 32 bit, now its using 2,41G out of 8G of my ram, and i play games

a 32-Bit OS can only use up to 3.2GB of memory.

to take advantage of the 8GB, you will have to use the 64-Bit version of Win7. Your product key will work on the 64-Bit version as long it is the same Windows version, meaning if you run Home 32-Bit the key will work on Home 64-Bit. You will have to do a fresh install.
 
While your O/S is limited to 3.2GB, your video card is also using memory from the last address down. If you have a 2GB GPU it will be using another 2GB. So in 32 bit you're actually using 5Gb, and under 64 bit don't be surprised if your O/S shows 6GB not 8.