I just did a brain transplant on a 2006 HP pavilion system that I originally bought in 2006. Single core AMD processor. I have not spent much over 300 bucks to do it but the time it has taken has certainly taken the value part out of the equation.
I was near a Frys Electronics visiting family and had about 30 minutes to kill. I did not have a parts list in my head, but knew about what I wanted. Not really the best way to buid your own computer, but the last time I ordered stuff online, it did not work correctly and sending it back made buying it at Fry's cheaper. I bought an i3 processor (4130) 4G memory (on stick of 1333, yah I know) and a full sized MSI motherboard. 10 minutes after buying the parts I realized I needed a micro ATX motherboard not the full size. When back to the store and returned the motherboard and got an ASUS H85 motherboard. After a 2 hour drive, I opened up the parts at home and found bent pins on the mother board CPU socket. So then I had make a 4 hour round trip drive back to the store and exchange the motherboard back for a new one.
So now, a week later, I was ready to build the new machine. Out with the old parts in with the new. I had replaced the PSU in the old system already, so I had all of the power connections I needed. However, the old CD Rom drive was IDE and my new motherboard only has SATA connections on it. I hooked it all up minus the drive and it went through post just fine. Yay me! But crashed miserably trying to boot windows7 (another earlier upgrade). No real surprises there. Amazon took only 4 days to ship me an IDE to SATA converter ($5.99) and I hooked it up in every imaginable way possible and non of them worked. Fail!
Plan B: The internet says I can make a bootable USB drive with my windows installation disk and a program from the Microsoft website. Good thing I have more than one computer that can read a CD Rom and has windows 7 on it. Otherwise I would be going to plan C, buying a SATA drive. After several attempts at getting Microsofts "Windows USB tool" to work and smashing my head against the wall because of it, I had to make the boot-able USB drive from the command line. That worked perfectly and was much easier then messing with Microsoft's free tool. I just took several hours to format and then xcopy all the files. The one problem I have with USB drive is knowing when to remove the drive. If you just leave it in, it will not boot from the hard-drive when it needs to.
Now my task was finished! Not! No internet connection. Now I needed to do the same process with the CD that came with the motherboard. Now I was finished! Well not really. It took another half day connected to the internet to download SP1 and internet explorer patches.
Good thing I enjoy the process.