|For best performance, an A64 should be run with DDR2-800. DDR2-533 or DDR2-667 will bottleneck the system. But according to HP, the max RAM speed your system supports is DDR2-667.|
Considering the memory controller is built into the CPU, I don't see how it can't support DDR2-800?
To answer your question, I suspect 2GB DDR2-667 would be better. Try it both ways & run some benchmarks for comparison. And just out of curiousity, which video card do you have? The onboard GeForce 6150 isn't very good for gaming so I'm guessing you installed a PCI-e video card?