Ok, so from what I understand, windows XP will only see a MAX of 3 gigs. Now I have dual channel ram, and so far have 2 sicks of 1gig. I would like to fill the other 2 slots, but can I do it with 2 sticks of 512? Or do I have to exactly match whats in the other 2 slots?

This is the ram I currently have....

http://www.newegg.com/Product/Product.aspx?Item=N82E16820145034

And these ones have the EXACT SAME voltages and timings, just different speeds. So Could I put these in with 2 sticks of 1gig, for a total of 3 gigs, in dual channel?

http://www.newegg.com/Product/Product.aspx?Item=N82E16820145040

As far as i know (at least with laptops) i think you are able to mix and match sizes of ram. I just bought 2 gb (2 x 1gb) of crucial ram for my i6000 and in the instruction sheet that came with it, it mentioned that if you are using different sizes put the larger one in the first slot and the smaller one in the second slot. Im just telling you what i know about laptops. I wouldnt take my word for it on pc's though. If you are daring enough i guess u can give it a try. Whats the worst that could happen?

...your pc could explode?

Unless you are doing massive 3d animations or something, you really shouldn't need more then 2gig Ram. If you check in Task Manager, you'll probably see that half your Ram is already free, therefore not necessary to waste money on buying more.

yeah thats true also. 2gb should be sufficient nowadays