superHard - there are some smart folks on these boards, but even they need a good deal of information to help in most cases.
For starters, post all of your system specs, including manufacturers and any existing overclocks:
1) GPU - which BFG model? Can you give a link so we can look at the specs?
2) PSUs - you've given the specifications for the Enermax, but you've also mentioned an Antec, which Antec?
3) Motherboard - what motherboard are you using? Does it have the latest BIOS update?
4) RAM - number, make, speed and timings?
5) CPU - e4300, fine, but are you overclocking it? If so, how much? (multiplier x front side bus)
6) Other - how many HDs and of what type? How many DVD/CD drives?
Also, describe exactly what the problem is, and perhaps be a bit more verbose than simply saying, "when the OS boots I get a 'your video card is underpowered error'." You may consider answering some of the following questions:
1) What gives you the error? Windows? A GPU utility?
2) Have you experienced any general instability?
3) How much is the GPU underclocked?
4) Have you tried gaming? If so, have you experienced instability?
5) When jumpering the other PSU over, which PSU did you use? Did you perform any tests in this configuration (gaming, etc.)?
As an aside, my wife is one of the brightest people I've ever met, she's also one of the most frustrating people for whom I have to diagnose problems. The reason: she's always jumping ahead too quickly, having made intelligent assumptions that sometimes turn out to be false.
Let the folks here lead you through the process, and you'll give them a much better chance to actually help you.
Cheers.