memory upgrade

you can but memory speed will be of the slowest memory module, that being PC2700, so in essence your PC3200 will be running at PC2700 speeds,

have you checked to see if your HP motherboard will accept a 3rd stick of memory, as it does it have a 3rd slot?

 

Hard to tell, sometimes more memory is better overall than actual raw speed if the modules are close in speeds... best option is to test with http://www.majorgeeks.com/download.php?det=4664 as it has a Memory benchmark tool.

Test with the 256mb PC3200 on its own and then pop in the PC2700 with the PC3200 and test again, if the difference in result is not to far apart then you may well be better with more at a lower speed than less at a faster speed... I did similar to a reletives PC a while back and it was worth adding the slightly slower memory to give the PC 768mb of ram.