These days in ATX PSU these voltages come from a single rail and have fixed amount of current.
Seems like they do not affect the PC functionality these days.
I know that USB port use the 5V rail but which computer components use 3.3V?
I am worrying because if I mount too many USB devices power hungry devices, my system might become unstable. I have 8 DDR2 FB-DIMMs and 2 Xeon CPUs.
3.3V is used by some minor chips like Super I/O, audio codec, etc, but nowadays the largest consumer is M.2 devices, like SSDs, that typically use up to ~10W each. PCIe also has a 3.3V supply besides 12V, that again might be used for some minor chips on cards.
3.3V is also present on older SATA power connectors, but was obsoleted in newer versions and therefor not guaranteed to be present anymore. It's not a problem as no modern device uses it anymore, and it's even likely to cause problems at it interferes with SATA's PWDIS functionality, which means your HDD will not spin-up with the voltage present.
Besides USB, 5V is used by SATA devices, sometimes exclusively as with 2.5" SSDs and HDDs, or partially for the control electronics in 3.5" HDDs, which additionally use 12V for the motor.
Anything else typically uses the 12V rail indirectly through it's own power conversion. For example your example of CPU and DRAM, both have their own power conversion.
Wikipedia says the CPU uses the 3.3 volts. I thought memory used it too, but a quick search shows it uses an even lower voltage.
If your USB devices are linked through powered hubs they will draw from the hub.
