1) You can upgrade the HDD to 128GB SSD, link for replacement HDD. http://www.youtube.com/watch?v=4HPzy1pZA30
2) You can upgrade the CPU too, depends on which one you have.
3) You can't upgrade the RAM with DDR3, because the MB only supports the DDR2 RAM, the top speed is DDR2 PC2-6400 (800Mhz, 8GB).