so many variables in this solution..
If it comes down to it, what is wrong with considering a whole new gaming platform (motherboard, cpu, gfx card ATX case etc.)
I definitely understand where you are coming from, i spent a few bucks on the upgrades to my machine. looking back, i wish i could have just done my homework and discovered a new platform instead of just spending a few bucks at a time for upgrades/band-aids on my wimpy p4 h/t xp32..
ok, now the PS issue, you definitley want to examine where your current PS is plugged into, i mean the big one, your motherboard.
Most modular PSs are capable of retrofitting with standard plugs. i guess the keyword here being modular. Next thing is, what is your graphics card scenario? does it require an additional plug from PS? Most gaming cards do require one or two additional power plugs in the 6 pin or 8 pin category.
Truth is, your system could only be demanding 200 watts, and a 1500 watt PS would be fine if all the connectors are properly fitted to their end sockets..
I would recommend a Corsair 650 watt TX series in the $120 range, it will serve 90% of the needs of the gaming enthusiast.
Also, the new generation of PS has a bizarre intake fan configuration. just make sure the massive intake fan faces downwards .