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The PSU comes with only ONE PCI-E connector for a reason...and the fact OP says the PSU doesn't even have molex connector means he get a 2nd PCI-E power, and using only one PCI-E on the 4870 is bascially playing with fire.To be fair, I've found Dell PSU's in the past to be quite good for power ratings. Often they understate what they are capable of, maybe its changed since the last time I had one though.
As for plugging in one connector, its not advisable. Couldn't tell you if it would work or not, but if it did, I'd be very wary about using it.