False, it will run at the same speed it does now, unless you add more ram, which it will run faster.
It won't bog anything down, except the clock speed of the RAM you bought. (It will decrease to the max your board supports.)
The RAM will run at the speed of the slowest stick. Yes, the RAM will run slower, but it will be faster than having 512MB of faster RAM, since things can be accessed much quicker in the RAM than in the hard drive.
