Are you sure you need more RAM? Often a slow system is a fragmented hard drive or one that is running out of space.
First thing I would do is make a backup of any files that are important. Next I would run this app to flush out the old cache & logo files: Disk Doctor. Also clean out any old files you don't need any more. The goal here is to free up space on your drive, ideally you want to have 1/4 to 1/3 of the drive free (smaller drives need more space).
Now the fun begins!
You'll need an external bootable drive to run Disk Utility from to check your internal drive. If you don't currently have such a disk here is the steps create a bootable external drive (a 16GB USB thumb drive will do).
Using your systems copy of Disk Utility delete the FAT32 partition and reformat the drive with GUID and create a Mac OS Extended (Journaled) partition. Then use the OS installer from the App Store to install a bootable OS on it.
Now reboot your system holding the Option key to select the USB drive. Run Disk Utility so you can check the drive & the permissions of your internal drive. That should improve things. To finish things here we should defrag the disk here you'll need another program like Drive Genius. Once done you should be zipping!
The other option here is just replacing the drive with a larger and faster HD or even going with a SSD drive.
If you really need more RAM your system can support up to 16GB! But before you alter things do make sure your systems firmware is up to date. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers Make sure you are at least at the levels listed here or newer.
Your system currently has 4GB using two 2GB memory modules. You can replace one module with one 4GB giving you 6GB of RAM, or you could replace both giving you 8GB. If you really need more than that you can replace out one of the 2 GB modules with a 8GB module giving you 10 GB and the last config is replacing both modules with two 8GB modules giving you 16G of RAM.
The modules you'll need to get: 1066 MHz PC3-8500 DDR3 SDRAM