Since you have 8GBs right now, lets talk about it in 8GB totals. Theoretically, you will only see a performance improvement by going to a setup with 2 4GB DIMMs. When properly installed (as per your motherboard manual) this will put you into dual channel operation. Going beyond this, using 4 2GB DIMMs will not improve performance any further because your RAM slots on the motherboard are grouped into banks that operate on each channel with two slots per channel.
If you were operating on the modern Intel X platforms, you would be provided quad channel operation, so using 4 2GB DIMMs would give you a further theoretical performance improvement. Since most of those motherboards have 8 RAM slots, we can take this a step further and think about 8 1GB DIMMs. This would still only leave you in quad channel operation and would no performance increase would be realized.
This article (http://www.gamersnexus.net/guides/1349-ram-how-dual-channel-works-vs-single-channel) does a really good job of explaining it. Based on the testing they did, you won't see much increase in gaming performance by switching to a dual channel setup. If you any kind of productivity or computational workload, you will see some improvement, but it probably won't be earth shattering.
Hope this helped!