Now that's funny.
The benefit of these "barebones" and "kit" PC bundles comes from the parts included being matched up to work with one another. While assembling a PC is something most anyone could do, selecting the proper components is somewhat more complicated.
Some kits and bundles do not include; 1) an operating system, 2) hard drive, 3) optical drive, 4) keyboard & mouse, 5) monitor.
Both of the links you provided appear to ship with almost everything you need apart from the OS.
Assembly of these bundles is straight-forward, requiring in most cases nothing more than a screwdriver. Seating the processor and applying the thermal compound are probably the most challenging tasks. Items like memory and drive cabling are designed so that they only go in the "right way". Power connectors are similarly configured.
If you buy a bundle or barebones that is not complete, re-using components is acceptable in some instances, but you may wish to upgrade. Your 7-year-old PC's hard drive will likely prove inadequate in terms of speed and capacity. It's old CD (DVD?) drive will likely also be not up to the task (IDE instead of SATA, 24X speed, no burning capability, etc.). Your keyboard and mouse may be serial and some motherboards today ship without these connectors.
If you have to buy memory go with a vendor that sells you the proper type. I would recommend buying it over the phone after you receive the kit and have an exact motherboard brand/model to provide them. Call, give them the motherboard specs and they sell you the right memory. Most include installation instructions/tech support free.
Any SATA 7200 RPM hard drive, USB keyboard & mouse and SATA 5.25" optical drive should work with your bundle. If upgrading a monitor, read the description to see if the motherboard/video card has RGB output. Some are digital-out exclusively, meaning your old CRT monitor may not work. You may wish to purchase an LCD monitor, as the prices have fallen dramatically. You may also wish to upgrade to Windows 7.
I recently bought a "kit" from Tiger to update my son's aging hardware and found it to be worth the price, and had no issues at all with them.
I built my PC last year (Intel i7) using Newegg exclusively, and as always received exceptional service and value.
Hope this helps.