RAM upgrade

I'm getting a 512MB ram upgrade to a comp. with 1 256MB card but I've heard it's better to have matching pairs.  Just a marketing ploy?

No, different sized modules may not work. BIOS (sometimes?) only detects the first slot if the sizes are different, windows mostly can use both though.

I have yet to find a computer where differently sized, even differently timed modules won't work. Of course, you have to set your timing to the slowest of the two or the fastest your motherboard supports, whichever is slowest. You might have problems if you wanted them operating in dual channel mode (which I doubt to be >>1's case) though.