If your 300W PSU doesn't provide enough power for that lot, it's seriously faulty, almost regardless of specs 😮
Calculating required power is quite simple, but you need to consider the different voltages separately. With an AT PSU it's simple, it delivers its power over 5V and 12V (ATX adds 3.3V, but the principles remain the same). There are also -5V and -12V lines, but unless you're doing something very odd (which isn't the case with the components you listed) they aren't relevant for calculating required poewr.
You can see what your PSU claims to supply on the sticker. A 300W AT PSU (by AT standards a monster) probably delivers 30A or more over 5V and 15A or so over 12V (and a 300W AT almost certainly also has a 3.3V AUX connector, which your motherboard probably doesn't use). Good PSUs can easily deliver the stated currents continuously. Bad ones can only do so briefly or not at all. Good PSUs can turn bad if their components - caps! - are degraded. It would help to know which brand and model PSU you have.
Then it's a matter of determining which components you have, how much power they consume and which PSU line the draw their power from.
- Rotating drives get both voltages, but only draw significant current on 12V for their motors. 10W (0.8A) per drive on the 12V line is usually a safe estimate.
- Motherboards, RAM and expansion cards without heatsinks are generally negligible in power consumption, but to be safe, budget 5W (1A) on the 5V line per card.
- Pre-P4/Athlon64 CPUs draw their power from the 5V line. Take the manufacturer's TDP figure for required power. In this case I can't find the info on amd.com, but did here: 15.4W or just ovefr 3A. Say 4 for safety's sake.
- Video cards can draw significant amounts of power, but by modern standards this is a modest card. Here it suggests the V3 draws about 4-5A at 3.3V. In an ATX system, the AGP slot is powered mainly from the 3.3V line, but I don't know where it would get its power from on an AT design. Assuming - worst case - that it's from the 5V line, and assuming a bad linear converter, that would mean 5A on the 5V line.
Adding that all up, I would say your system needs:
- 4x 0.8A = 3.6A / 40W on the +12V line
- 4x 1A (board+cards) + 5A (CPU) + 5A (Voodoo) = 14A / 70W on the +5V line.
= 110W total
That means that some lowly 180W PSU should probably be able to do this with buckets of power to spare. It also means that if you have problems with this setup and your current 300W PSU, either that PSU is seriously failing or your problems have nothing to do with PSU in the first place. You can figure that out by testing with the 180W PSU. If you are lucky your motherboard BIOS can tell you the voltages for the various lines. Have a look there (or measure with a voltage tester). If your PSU can't deliver what your components require, voltage will drop on the line that is in trouble.
Note that modern (ATX12V 2.x) PSUs are designed to deliver most of their power on the 12V lines, as that is where modern CPUs and PCIe GPUs get their power from, so they have far less capabilities on the 5V line. That means that more powerful pre-P4 setups can hit problems there - but yours is so low-power it wouldn't. I checked my least powerful ATX12V PSU, a 250W Seasonic minnow. It can "only" deliver 16A on the 5V line. That is half what a 250W AT PSU would do, but even so it would suffice for your setup, so a decent modern 300W would have no trouble. Also, if your problems are PSU-related, the fact they are all associated with disk drives suggests it would be the 12V and on a modern PSU that is definitely NOT where to expect trouble 😉
