Depends on your operating system (and what you're trying to do, to some extent).
For most home users, using Windows XP, Vista, or 7, getting more RAM shows an improvement up to (about) 4 GB (32-bit OSes can't fully access all 4 GB). After that, there isn't much benefit unless you're doing audio/video/photo editing or similar tasks (and using a 64-bit OS).
Once at 4 GB, the average user might see small (1-2%) improvements with faster RAM. Note that typically lower CAS latency will provide more benefit than increasing the clock speed of the RAM. For example, CAS 6 1333 MHz is faster than CAS 8 1600 MHz RAM.