I reread your post a few times and am not completely sure what you're asking, but I'll give it a shot.
No matter what you do, your motherboard will downclock all of the ram to the slowest sticks.
You CAN upgrade adding 2x512 sticks of PC 3200.
You CAN upgrade adding 1 stick of 1GB ram, but it will not work in dual channel, if you have that option.
Assuming you do have dual channel, you can later add another of the same stick in and run a total of 3 gigs, and all sticks will be running in dual channel.
<b>I'm an unhyphenated American.</b>
Intel Q6600 @ 3200 1.4v
Abit IP35 Pro "The Snake"
2x2gb A-Data @ 800
Diamond HD 3870 512mb
Great FORD TRUCK