raihan4 :
[The 750Ti] . . . needs 20 amps on the 12v rails and a 400w PSU as a minimum.
I'm pretty sure this is false unless you get a special version of the card that includes power plugs. I have a 750 ti, and it consumes
very little power. It runs entirely off the PCI-e bus, and there is no power plug running to it from the PSU. A PCI-e bus delivers up to 75 watts only. Plus the card's temperature doesn't noticeably increase under load. It's extremely power efficient and cool - exactly what you're looking for. In fact, you can even read independant reviews from AnandTech (one of the most reputable hardware reviewers there is), stating that the 750 ti is "
The Most Power Effecient GPU Ever Built." (click for original article and review).
Aside from that, however, OP: I am pretty sure that you can upgrade the power supply in your Dell Inspiron 570. It is just a tower. Open it up and look at the power supply to see whether there are screws holding it in. If so, unscrew them. If it comes loose, you can replace it. I can almost 100% guaranty you can swap it out. How would Dell have assembled it, and how would they fix it if you sent the computer in for PSU repair? When Dell says that it "can't be upgraded," they probably mean that you just can't choose which PSU you want when you buy a new computer in the same was that you can choose (or "upgrade") to buy 16 GB instead of only 8 GB of RAM when you first purchase the machine.