Hard Disk. I recently saw a new computer with a 6 gigabyte (GB) hard drive. But before that, I haven't seen one with less than 10GB in over a year. Not long ago, I bought a 100 GB hard drive for $130. It's common to see a 50-60GB drive for $50 these days. Upgrading a hard disk can be as simple as adding one, or as complicated as "ghosting" the old drive on to the new one, complete with cables to switch and jumper switches to move.
RAM. You really can run Win98 with less than 128MB, and 256 is good. With Windows 2000 and XP, 512 is the least I'd recommend. Often you can just plug in more DIMMs chips to add memory, but the motherboard sometimes requires that you check the speed, and that all the banks have the same amount of RAM.
Video. The Video RAM is possible bottleneck for a computer. If you think about it, a 600x800 screen running 30 bit color has 480,000 x 30 bits (or 14,400,000 bits) for a full screen. If your video card has only 8MB of ram, the screen won't fit into the ram. Consider the newer cards, with 16 or 32 MB of RAM. Upgrading video usually means removing and replacing the card. Be sure that, if you don't have an AGP slot on your motherboard, that you try to be sure that you get a card that can plug in to your computer. Most video cards these says are either AGP or ISA.
CPU/Motherboard. The processor chips that are for sale today are all fast enough for almost any use. If you feel like you need a faster one, though, you may find that you need a different motherboard as well. The speed of the bus (etched on the motherboard) will help users decide which boards are upgradeable and which are not.