The subject of rewiring and not damaging anything is outlined in the first two of my preceding links -- you just need to have the aptitude, patience, attention-to-detail, and basic wiring skills; or have a skilled friend help you; or pay a professional tech to do it for you [a skilled pro could probably do it in under an hour if you provide a kit with all the necessary components].
I personally would not recommend the dual PSU solution (keeping the original power supply and adding-on another power supply), because:
1) I would not trust the stock PSU to last much longer than the warranty period (and maybe not even that long), especially under heavy use. And a power supply failure is not unknown to damage other components of your system (especially if the PSU that fails is a cheap one).
2) You have the problem of fitting two PSU's into the case, with a custom fitting challenge (and air-flow consequences), or you have to mount the second PSU externally which has its own set of disadvantages.
3) My guess is that a single PSU would be more efficient than a dual PSU setup, with lower net equipment cost, lower electrial-power consumption, and less total heat dissipation.
4) My guess is that a single (good-quality) PSU would be quieter than a dual PSU setup.
About the only advantage of the dual PSU approach is that it might be easier and faster to implement.
BTW once you upgrade the PSU (and add a video card to match your gaming goals), I think you'll have a decent machine for the money spent.
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