I have upgraded the memory in my pc from 1gig pc4200 ddr2 dual channel to 2 gig pc6400 ddr2 dual channel is better to leave it at the 2 gigs or put the original ram back in as well and have 3 gig? Will all ram speeds be decreased?

Might consider taking it for a test drive and see for yourself..

Alot depends on what and how many apps you run....

But mixing ram will make ALL ram run at the 4200 speed.

In addition, check in the BIOS at what FSB your system is running at; because your memory is rated at ps6400 doesn't mean that is what it is running at, like Steve said. Just to detail it our more, the whole system will default to the slowest speed of your CPU, memory and FSB. That's why you want to always spec your computer so that all those components run at the same FSB and you do not have what is called a bottleneck. So if you mix pc4200 and pc6400 memory, your whole system will run at the slowest rating, in this case pc4200. BUT the memory will work fine, just at a slower speed than you want.