So, are you saying I should go with the lm317 over the lm2576? Arent the lm2576 used in those buck converters?
It's up to you and the confidence you have in your skills. Since you started simple, and this is the beginner section I imagined you were after a simple starter project.
Yes, they are used in buck (==down) converters, that means the output voltage will be lower than the input one.
I have seen posts on this forum before telling people not to use ATX supplies as a bench supply.
Safety, accuracy all come out as reasons, but not here.
Why not?
If using an ATX PSU involves modification of the internals (holes and wiring changes) and removal of the ground connection (as seen in some horror video on YT), I'm all against it.
If the supply is simply used as the input source for a regulator stage as seems to be the case here, I do not see major problems, when done carefully.
One of the things that might be overlooked is that all the output voltage are referenced to earth ground, that's why I pointed it out in my first answer.
Another point is that an ATX supply can provide a lot of current, especially on the 12V rail, though good ones have protections (don't know about the OP one).
The post regulator (LM317) section should at least be fused appropriately.