Help understanding ram

Hi, I added another stick of 512mb ram and it works fine if I take the old one out and coincidentally, the old works fine without the new. I have been researching and the more I read, the more confused I became.
However, I did notice that the old one is 200mhz and the new is 400mhz. Is this the reason that I can't get my pc to boot up at all with both installed? Any insight would be greatly appreciated, I am totally lost.

Here's my specs:
Compaq Presario- 2 gig AMD Athlon 64
Board: ASUSTeK Computer INC. Diablo 1.xx
Bus Clock: 200 mhz
BIOS: American Megatrends Inc.
512 Megabytes Installed Memory

I'm not sure what other info I might need to give, I hope that's it. Thank you in advanced.

 

If you install new ram in your system its best to have the same type of ram you already have. This will ensure you that you wont have any conflicts.