Question : Replace pc4200 with pc6400

I have a Vaio vgn690 I was getting some error messages that suggested that there was a problem with one of the two pc4200 512MB mem sticks. I happed to have 2 - pc6400 1MB sticks lying around so I popped them in. it seems to run great. I realize that it may not be running the pc6400 sticks to full potential, but is their any reason I shouldn't continue running them?

Answer : Replace pc4200 with pc6400

Depends on the motherboard whether such RAM would typically run on the board - per crucial.com your model of laptop apparently is only "supposed" to support PC5300.  You don't risk anything by running the higher grade that you currently have in.  It will either work, or it won't - in this case, you've found what is likely a very handy undocumented workaround.