A PSU fan will start even if the +12V rail puts out just 6V and the other rails are dead, so you really need to measure all of positive voltage rails with a meter, preferrably while the PSU is under realistic load, such as a known good motherboard, because some PSUs will measure OK with no load but droop like crazy if they have bad capacitors. I'd probably try another PSU (but not buy one).
Is the CPU heatsink plastic retainer frame in good shape and able to clamp the heatsink on both sides? Sometimes one of the retainer frame's plastic hooks can break off and make the heatsink lose contact with the CPU. Most motherboards will shut down if they sense overheating from that (maybe not in time), but I believe Socket 754 CPUs have built-in overheat protection to prevent any damage.
It's possible the motherboard shifted a bit and has shorted to the case, but the only way to know for sure is by removing the motherboard and inspecting it. Check the mounting holes, top and bottom, and add insulator washers as needed. Also check the corners because motherboards and cases can flex more than you'd think, and lack of support within 2" of a corner can allow a short. If you operate the motherboard outside the case, be sure to take appropriate anti-static precautions.
Photos of the Asus K8V-MX motherboard show that it's made with a mix of capacitors by Panasonic ("M" surrounded by a rounded square, tops have a curved "T" stamped into them) and Chemicon (probably brown shrink wrap, tops stamped with 3 even pie cuts). Panasonics tend to last a long time, as do most Chemicons except models KZJ and KZG. Worse, they're known to fail despite showing no physical signs of damage. OTOH about all other Chemicon models, including KZE, last a long time. I noticed that some versions of this motherboard have Panasonics in place of some of the Chemicons, maybe because the latter were wearing out too fast. The caps to worry about most are those in the CPU voltage regulator (between the CPU socket and rear of the motherboard) and the caps in the memory voltage regulator (on the opposite side, next to the floppy disk connector), and sometimes the caps around the North Bridge chip (2nd biggest heatsink on the motherboard) wear out fast from that chip's heat. If you buy new caps, be sure to match capacitance, diameter (not much room on motherboards), and ESR, and meet or exceed voltage, temperature, and ripple current. DigKey and the PC Motherboard Capacitors Store are good sources for small orders of caps because they don't charge much shipping for them, and they sell quality products (PCMCS also sells junk brands, like OST and Teapo). Stick with brands like Chemicon, Panasonic/Sanyo/Sun, Nichicon, and Rubycon. Don't try soldering/desoldering a motherboard without first practicing on a junked circuit board made of at least 4 layers of copper.
Few new motherboards support PATA IDE, only SATA IDE, but there are plug-in PCI-E cards that provide both a pair of SATA ports and one PATA port (can run 2 PATA drives off one PATA port), and there are adapter cards that plug directly into PATA IDE drives so they can be connected to SATA ports. However with the latter, many are incompatible with optical drives -- some won't work with them at all, others will let you read and write them but not boot from them. One PATA-SATA adapter that does seem to work with everything is the 2-way model sold by Meritline, anywhere from $3 - $5, shipped, but it's poorly marked (easy to connect the cables wrong) and easy to plug in the wrong way (its PATA connector's plastic body is too small). BTW if you need to use a SATA disk with a motherboard that has only PATA ports, avoid the adapters with a plastic housing because the housing makes it too wide to fit some motherboards.
