Hi and welcome to Tom's forum.
If you upgrade to 2x1GB PC2-6400, the RAM only can will run at PC2-5300 because of the 2x512MB PC2-5300, the memory controller can't run at different speeds.
I'd go with 2x1GB PC2-5300 for a 3GB total at the same speed and you will see a improve in performance.
I have a 620 with 2x 1GB PC2-5300 and 2x 512MB PC2-4200 in their properly paired slots. The system is running memory at 266 MHz. HOWEVER, when I remove the 2x 512MB sticks, the memory does NOT go to 333 MHz. Why not?? I want to replace the 4200s with 5300s but not if it stays slow!
Do you know why?