Well...I did some search of your motherboard, you only have 2 memory slots, but i couldnt find out the max. speed your motherboard support, It only shows me that it comes with the speed DDR2 533. Therefore, i suggust that you stick with the speed DDR2 533 and get TWO 1 GB RAM.
Moreover, if you get speed with DDR2-667 or DDR2-800, you motherboard still can work too but it is only run up to DDR2-533 speed. If your motherboard support the max speed is DDR2-533 let say.
I think your motherboard could support up to DDR2-667.
In addition, I suggest to use either Kingston Technology or Corsair memory because those 2 brands are the best for fast performance and stablility.