Welcome, Newcomer! First, you should understand that mixing RAM will often result in unfavorable experiences, such as Windows Stop Errors, a.k.a. BSOD. If you are bent on using all of your RAM, just know this:
1. Your DDR2-800 will have to be down-clocked to DDR2-667
2. both kits will have to operate at the higher voltage between the two kits
3. both kits will have to operate at the looser timings (X-X-X-X) of the two kits
The above limitations are apparent simply because you can't force RAM to operate better than their best. Meaning, the PC2-5300 (DDR2-667) can't be used at a frequency higher than 667 MHz; it's physically limited. Similarly, if one kit requires 1.7 volts to operate, but the other kit can't handle voltages higher than 1.5, you'll end up over-volting and damaging the lower voltage kit.
In short, mixing RAM is not a good idea. That being said, just use your DDR2-800 2x2GB kit, which is more than adequate for XP; even the x64 architecture.
Edit: I should note that the examples on voltages are just examples, as I don't know what the voltage requirements are for your specific sticks.
Ddr 2 is pretty cheap. If you really want to run the extra sticks of RAM, just buy a similar set in comparison to your current. I had 8 gb of ddr2 in my old setup. The more the merrier, in my opinion. I personally thought 4gb wasn't really enough. It is plenty, but you only have to satisfy yourself.
Also, something you should note: More ram at slower speeds is generally better than less ram at higher speeds. This is what I've come to learn, I'm sure there are people to debate the topic.
I'm running almost 5gbs of my ram, with Starcraft II minimized, 4 screens on firefox up, apple software downloading an update, a few pc utilites, and spybot on.
But ~2 gb is being used by starcraft II minimized.
(I'd get the extra ram, especially if it's cheap!)