Combining RAM -- help or hurt performance

The two would default to run at the lower speed and timings.

What OS?

KK - the reason I asked that question is that with XP (or any 32 bit version of Windows), you need to be aware of the address space limit: You have 4GB worth of addresses that need to be used to handle everything you put in the case. This includes Bios, communications, PCi devices, and anythign that maps I/O into memory [/i](read: a portion of your video card's memory counts, too)[/i].

So: (1) If you install 4GB of RAM, then the OS will "see" less than 4GB (a little over or under 3GB) depending on your configuration. The reason for this is there is more than just RAM that needs addresses, and anything that doesn't get assigned an address with the OS is rendered non-functional because it can't be communicated to/from. So RAM gets assigned last because that way everything else in your computer works. If RAM was first, then nothing else would, yah?

As a practical matter, roughly 3 out of 4 isn't GB bad, since it's still a 50% increase and it's still best to run a matched set.... But understand that if you wanted to use all 6GB of RAM, then you will need to change your operating system to one that has enough address space to cover it all. (Read: 64 bit)

4GB works well either way - i.e. with a 32 or a 64 bit OS.

At this point, I would recommend you see what it's like with XP - since there's an existing installation to worry about and that's the least disruptive solution.