Hi, I would like to know about DDR SDRAM speeds.

Am I correct in thinking that all of the RAM cards, as a whole, only run at the clock speed of the slowest RAM card? If so, I have 2x1GB @ 400mhz. Will it increase or decrease the performance to add 2x512MB cards that run at 330Mhz?

You have two considerations: a) speed b) RAM in system.

For speed, you will run your memory @ 330 Mhz. So, you lose a little speed - should make almost no difference in performance.

For system, having another 1 GB of RAM will mean less swap file usage. This will benefit more Windows 7 & Vista which are memory hog OSes.

Windows XP ( & older ), Linux, BSD, Haiku run good with 1 GB RAM. You already have 2 GB, so going to 3 GB would have little impact on these OSes.

Edit: In addition, adding more memory dimms to a system may make it unstable. Good quality RAM and/or motherboard will reduce this from happening.