I will respond to your equipment questions one at a time.
Motherboard: Almost all motherboards work just fine out-of-the-box. Few rarely work normally, then break a year later, therefore a one-year warranty should suffice. It is much more common for a motherboard to be bad the second you install it, in which case, Newegg will exchange it and ship you a new one at no charge. Note that warranties cover such things as design or manufacturing defects, not "Acts of God" or problems caused by the user. This means that if you kick the PC and snap the motherboard, or if a water pipe bursts and floods the place, ruining the motherboard, a motherboard warranty will not help.
Power Supply: Sure, it can wait, especially since you don't need to install a discrete graphics card.
CPU: Both the Gigabyte and the Asus are very similar motherboards, in fact, they share the same chipset. The only differences are the price and warranty. That CPU, along with all the Athlon II's and Phenom II's, will not work in your current MSI motherboard. Additionally, your current Athlon 64 will not work in any AM3 motherboard due to differences in memory standards. Replacing the motherboard and installing a new, modern CPU will drastically improve performance. Your old MSI motherboard takes AM2 CPU's and AMD stopped making them a while back in favor of AM3 CPU's like the Athlon II or Phenom II.
RAM: DDR2 will not work in motherboards designed exclusively for AM3 CPU's. 4GB is more than enough to meet future needs, that is why I chose it over a 2GB kit.
HD: A higher RPM will equal improved performance, there are some drives out that spin at 10,000 RPM. Note that drives which are platter based, like almost all drives in use, wear out and will typically start failing after 5 years. There are also solid state drives (SSD) that have no moving parts and are far faster than the platter based stuff, though they are very expensive.
OS: As tg1911 pointed out, OEM operating systems are tied to the motherboard. You have one such operating system. Therefore, you need to buy Windows again. Microsoft defines a PC based on its motherboard, so if you change a motherboard, you effectively have a new PC. OEM editions are paired to the motherboard and cannot be installed on a new PC, which means replacing the motherboard breaks the license. The good news is, you're getting a new OS that runs better, is more secure, and looks nicer than Windows XP, so it is not all bad. Remember, you need a 64-bit OS to use 4GB or more of memory, that is why I specifically chose the 64-bit version of Windows. There is no difference in cost between 64-bit and 32-bit editions. There are lower cost options if you don't want to spend the $100. You could try Linux since it is free, but unless you know what you're doing, you may become frustrated. If you do go this route, I suggest
Kubuntu. I used it, it reminded me of Windows, but I ended up removing it due to driver issues. Also, if you know someone in college, and their university is part of
MSDNAA, they can get a free copy of Windows by simply downloading it from a website. There is no technological barrier to installing XP on this machine, only a legal one since you are breaking a license you agreed to. Installing XP on a new machine is just...silly. Especially when you save money by not getting a new power supply.
This post has been edited by DJBPace07: 17 March 2010 - 01:04 AM