You can upgrade your boot disk to a 10,000 rpm disk, or even a pair of 10000 rpm SATA drives inside. You can get an 8-channel SATA card and have a stinking fast 4- or 8- drive RAID array externally for your video and photo scratch disk. You can upgrade your video card to the fastest one available. You can upgrade your DVD drive to the best one available.
BUT you can not upgrade your processor. All of the things above will make speed improvements to your system.