Question on adding ram

I have 512 MB of RAM right now. Can I just add a 1 GB stick of RAM in my second DIMM slot? So that I have half a GB in one slot and a full GB in the second? I have a Compaq Presario 3100+ AMD Sempron computer running XP home.
The RAM is DDR 400 (PC3200)...

for both sticks.
And do I have to put the larger module in a certain slot?
Thanks.

Theoretically, as long as the frequency of the two modules are the same, it should work. Unfortunately theory and practice doesn't always come together and I have in the past had instances where they work and don't work (depending on the manufacturer of the motherboard)

As an IT best practice for older motherboards and computers it is advisable to get two identical modules in the two slots (that are compatible with the motherboard), but I would suggest you take the computer to an authorized HP/Compaq reseller and ask them to upgrade the memory.

Reason:
If you ask them to upgrade the computer to 1 1/2 Gig memory and they install the wrong module, they should essentially supply you with whatever solution you require to end up with what you want, and they should also inform you if it isn't possible. That way if they install something and it doesn't work, you can not be held liable for the cost of the modules as they are essentially "specialists" and should have known if it wouldn't work.

Hope that was somewhat helpful

As long as it fits - physically - it should be fine. When I added ram to my last desktop computer, the slots were identical, however, one was harder to get at.

The board might handle it, not sure how much it can handle. It really depends on the model of the computer, not just the processor speed. I would guess that you could safely double the current memory in the thing, but I am not sure if you will be getting that machine up to running programs like Vista.

Thanks to all who answered this.