You can upgrade the RAM in pretty much every notebook, even the original orange "toilet seat" iBook G3 used standard PC100 SO-DIMMs.
Graphics upgrades are rare. Some notebooks, generally the thicker, larger "desktop replacement" models have upgradable graphics. The cards themselves are usually really expensive unless you're buying an older second-hand used card via eBay. BIOS and firmware issues also come to mind. I have a friend with a 2-year-old Dell notebook with upgradable graphics. Dell never made newer graphics cards for it, even though some newer Dell models uses the exact same size graphics card. He tried to upgrade the graphics, but the machine wouldn't even POST due to some BIOS/firmware imcompatibility.