If you've decided to upgrade your system to something based on Intel's Core 2 CPU or a newer AMD processor, you will probably need to upgrade your memory as well. At the DDR2 800 level, there are plenty of choices - high latency ram that can reach high FSB rates, or low latency ram that runs near stock, but does so with high performance.
The best choice for most is to find something that has the lowest latencies possible. Kingston's best modules in this category is their HyperX PC2 6400LL, which is rated at 4-4-4-12 at 2.0v. It can overclock too!