Not a significant improvement if you simply intend to switch the kits, it would not be worth it. If you are adding that RAM you will have to upgrade to 64-bit, I think they should be compatible, they will probably just run at the timings of the slower kit.
You might see an improvement if you use Dual Channel Mode. Just make sure you buy a Marching Pair of Ram Modules also referred to as a KIT. Basically these are identical Modules and are sold together as a pair.
Other than that, you won't see any significant improvement. You will mostly only see improvements when you increase the amount of Ram.