assuming that you play games and don't mind ocing a bit, buy 2GB, it'll come handy when gaming and you can always oc 667 to 800 (depends on the ram and mobo of course).
1Gb 800mhz good for overclocking an minor games like solitaire, pong and Return To Castle Wolfenstien. 2Gb is better with newer games like BF2, Oblivion and Supreme Commander.