If you haven't looked at power supplies, I suggest looking at Seasonic's S12II series. They're efficient (they're 80+ Bronze, but some models come close to Silver), very inexpensive (the 520 watt version only 80 Australian Dollars at Newegg, you can probably find it cheaper elsewhere), and are downright solid. They won't play well with Haswell, but you don't have Haswell (it can be remedied by turning off certain power states with Haswell anyway). They also have a 5 year warranty when bought new (not the 3 year warranty on the box--they changed it right after it was released) and despite what it says on the power supply, a single 12 volt rail (this is good for a 520 watt power supply, but with something like 700 watts you should start to consider dual-rail).
The M12II Evo is an improved (slightly) variant of the S12II that is fully modular. Non-Evo M12II are out there but are discontinued, and are basically partly modular S12II power supplies with no other alterations.
Note that a lot of good quality "cheap" PSU's are based on the S12 II platform--Antec Earthwatts 520 is basically a slightly customized Seasonic S12II 520, for instance. Earthwatts 520 might actually be a better buy if you can find a new one, as they are cheaper due to lacking a power cable. If you have a solid power cable that works with it, you can use that and save some cash.
Like others have said, you do not want to play around with cheap or old power supplies. Dell's power supplies are only efficient for what they already have inside their systems--the 5450 is a power sipper, but anything even a little more powerful is going to suck power out of that PCIe port (about 70 additional watts if they're not overclocked--if they are, don't get a card without at least a six-pin power port, because they can ruin your motherboard).
The 9xx GTX series, as mentioned above, will be good for you, but be warned the 970 will be slightly short on RAM in modern games (it is actually a 3.5GB VRAM card, not 4 GB as advertised; this is right on the edge of viability for 1080p in newer games, but it did lead to a false advertising lawsuit when the GTX 9xx series was dominant). Because of this, however, they are/were not often used by miners, who preferred cards like the 980 Ti for their higher VRAM and compute power (AMD cards were far more dominant here for a while, as they had better compute performance than GTX cards at the time. Don't by used AMD cards, as AMD still gets used a lot for mining.)
