I've been an AMD fan since 1999 when I bought a K6-2/400. I went 64-bit in 2007 with an Athlon 64 3200+. Then I went to and Athlon 64 X2 5200+ and a Phenom II 940 BE (quad core, 3GHz).
The 64-bit chips were installed in an ASUS M2N-SLi Delux motherboard with (initially 1GB DDR2 667) 4GB DDR2 800 RAM.
Each upgrade produced a very noticeable and exciting boost in performance, and having multiple cores to play with is cool. The Phenom II 940 BE is a 125W CPU, quite hot but the fan wasn't too noisy. The motherboard eventually gave up (the capacitors split open) so I replaced it with an ASUS M4A77D motherboard, which so far (after about 18 months) been very good.
I don't believe in spending vast sums of money on the absolute top-of-the-range CPUs but occasionally I like a new toy to play with. So I went looking for a Phenom II X6 to put in this motherboard, which supports most of them with a BIOS upgrade.
Unfortunately, I left it a bit late to buy a Phenom II X6, and the only one I could find at a reasonable price was a 1045T which "only" runs at 2.7GHs but it has turbo core (a kind of frequency scaling) which means that it can overclock one (or maybe 2?) of the cores by up to 500MHz if the others are not busy. The good thing is that this CPU is only 95W so it pumps out less heat and uses less electricity. As a rough estimate, the 10% lower clock frequency is compensated for by the extra two cores fairly well so that overall, on something like SETI@Home, it should be 45% faster.
I wrote a little program to do some very simple number crunching and timed it. It does seem to go a lot faster when none of the other cores are in use. This wasn't a very scientific test, so I'll have to investigate further.
My machine only runs Linux, so to flash the BIOS I used a utility called Flashrom which you run from Linux as root. I downloaded the source and compiled it (on Slackware64-13.37) and it Just Worked(TM). Even although the M4A77D isn't listed as being supported, using lspci I saw that the chips on the PCI bus corresponded to those on their supported list. I used it to read the BIOS from flash a couple of times, and compared the binaries by eye using hexdump -C (to see whether they looked sane) against the uncompressed BIOS file to be installed which I got from the ASUS website.
So I took a deep breath, wrote the new BIOS and rebooted...
It all worked, so I installed the new CPU and away it went!