X-bit Labs researched whether you should spend money on extra RAM or a solid state disk once the CPU and graphics card of your PC perform up to your expectations. The site concludes installing a SSD is the best way to get more performance once your system has 4GB of RAM.
The standard amount of 4 GB is enough for most situations and adding more memory doesn’t bring any noticeable benefits. You can only get a positive effect from such an upgrade if you've got a very slow HDD in your system. The SuperFetch technology which uses system memory for look-ahead data caching may make your computer more responsive then. However, it would be wiser to spend the money to improve your disk subsystem and get much better results.