Best upgrade? Buy a new PC.
But if you don't want to replace a MoBo then,
best CPU: Intel Core 2 Quad Q6600
best RAM: 4x 2GB DDR2 1333 Mhz (for 8GB of RAM, your OS needs to be Win Vista 64-bit or newer)
best GPU: any, provided you also buy a PSU with sufficient wattage for GPU