I re-read the thread and see that I probably have misunderstood what you meant.
I thought: you already have one 512 MB RAM stick at 533 MHz; you want to add another 1 GB RAM stick at 800 MHz. Now this would be a very bad idea.
But it seems like you want to replace the 512 MB RAM stick with the 1 GB RAM stick. If this is the case, by all means, go for it.
Sorry for the confusion.
Yes. I will remove the 512mb stick and put in the 1GB stick. There are some on ebay that are around $10 with free shipping. Do you have any experience with those or recommendations?