The internet description is wrong but it turns out that the PSU is the exact same model when I saw the info written behind it. The store owner said that it's 450W.
I don't see any unused additional connectors that are left. All cables that are attached to the PSU are filled and connected to my components.
So, can I add a new additional cable to my PSU?
if that sticker on PSU is same as in your PC...well then that store owner kinda lied to you
12V rail is what u need for GPU and CPU, it has just silly 24amp (288watts)
that doesnt mean u can run 288watts on this rail, its just peak capacity..u usaly go 50% during gaming with few bumps here and there (like when u start up pc, there is huge spike)
but on another hand looking at your 5V rail which has 30amps (150watts)..well this thing is pretty aged, because in 2003 was introduced ATX revision 2.0 when 3.3v/5v rails got reduced amps
so psu is made for pentium 4 before pci-express era (2003-2007)
usualy those cheap PSUs have 2year warranty
while premium priced PSUs have 10year warranty
in any case ure way past 10year life time and u want to push it hard?
well if something wont melt, ull just get few fireworks there