Slow down in comparison to what? Your current amount of RAM? No. 2x1gb ddr2-667? Yes, but the main thing is you have more memory. Unless you're ocing, I wouldn't worry a heck of a lot about speed. Another reason to go 2x1gb though is for vista expandability, but that may not be an issue for you. How much it would get slowed down, I'm not sure. Probably to 410mhz since ddr400 in all 4 slots will run @ 333mhz. That's all I know for certain.