Hard disk size is virtually unlimited in computers-however, if your computer doesn't support 48 bit LBA, you are limited to 137.4 GB IDE drives.
RAM amount and clock speed, CPU clock speed and number of cores and amount of L1/2/3 cache, and Hypertransport speed, CPU-northbridge frequency, all play a part in the "overall" speed of the computer. If you are a gamer, you will want to focus on the graphics card too.