Out of Pod Experience

@Kaaeliaa : There is absolutely no issue with using a microATX board in most mid towers, at most you need to switch around the holding pins, I have not seen even a single mid tower that could not mount a micro ATX board yet.
Ventilation would only start potentially being a problem if the case would be filled with cables and high-power-usage components cluttering airflow in areas where it matters, but this should not be an issue either given his build, quite the contrary.
Also, just the CPU plus a regular ATX board would cost him more than the entire build he initially proposed, while not bringing him much of a benefit (as in, the former dirt cheap build would have done its job already).


I say the PSU doesn't matter THAT much, especially if you do not live in a notoriously bad power quality area... at least not enough to justify spending cash on a decent PSU when you already have one (no matter how lousy) which you could not sell either way (or if you could sell it, you'd get at most 10$ for it).

A 6770 power draw is under 110W normally, and 75W of it comes via the PCI-E slot, so the crappy MOLEX-to-PCI-E adapter only has to handle a lowly 35W extra (and that from two separate connectors, so only 17.5W per MOLEX), whereas the MOLEX max specs are 132W per pin max with a 30C rise... in comparison, how much he needs is a drop in the bucket and will barely affect the connectors.
If he got a 200+W power draw video card, things would look different, but again, given his budget, even the 6670 is sufficient.

Even if his "rated 550W" PSU is indeed a POS, the power draw of a 6670 is only around 66W (with the rest of the machine not even getting close to 200W extra needed), so even if it would be an OLD POS claimed-550W one that only delivers 450W at best in a stable fashion when new, and decays to 300W when much older, it would still more than suffice.
What happens when you under-use a PSU is you lose power efficiency, so you end up consuming a bit more electricity than you could have consumed.

Sure, there's a chance everything might blow up due to a power spike or something else like that, but there's also a chance it will only blow an internal PSU fuse, or simply fail gracefully and get fixed by a plug out, plug back in, reboot.
The chance of stuff going horribly wrong is small enough to not justify the added expense when every dollar counts because you're doing a budget build and you already have that component purchased... unless you KNOW power quality is exceedingly poor in your neighborhood, and even then, it's a calculated risk.