Just a general question, how do you know when you're getting enough power or when you need a more powerful PSU, especially in a machine too vintage to be covered by something like that Coolermaster Online Guide that some videos recommend?

I ask because I have a Dell GX260 which has a Geforce 4 ti4200 in it and so far has worked just fine with the stock Dell PSU that has been in the case (which I think is 300watt). Should I be satisfied with that, or would I get better performance out of like a 450watt? Or does it even matter? There's surprisingly little information about this, for how much PSUs seem to be an important factor in PC building.