When a Mac Pro with ECC enabled gets a single-bit Memory error, it is corrected on the fly. Your Mac does not even slow down.
When it gets an uncorrectable error, it halts with a kernel Panic to avoid poisoning your data.
That peace of mind is worth a few extra bucks per module. Do not defeat Error Correction Code memory. That is a false economy, and is more extreme as your memory gets larger.
The airflow inside a Mac is lower than in many PCs, by design. This allows the fans to be less than deafening. If you install Mmeory that does not have the proper heatsinks and Apple-approved temperature sensors, they will get too hot. This will make the fans run faster and louder. It will also lead to premature memory failure.
What model Mac Pro do you have?
Memory for the 2008 and previous are expensive, but not available in non-ECC.
Memory for the 2009 and later are not that expensive.
Graphics cards with Apple firmware show a picture from power-on.
PC cards flashed to Apple firmware stay dark at startup, and do not show a picture until the login screen. This makes booting from an alternate source of software and other debugging difficult.
PC only cards do not show a picture under Mac OS X.