According to some latest rumours GT300 performance is about 10-20% better than 2XGT200.
http://translate.google.com/translate?u=http%3A%2F%2Fbbs.chiphell.com%2Fforumdisplay.php%3Ffid%3D4&sl=zh-CN&tl=en&hl=en&ie=UTF-8
So 384-bit in GT300 and 3 times faster SP (~3 Tflops?) than GT200 ones...