Looking at the specs of the XPS 710, it comes with either a 750w or 1000w PSU, so that looks good. It also seems to have two PCIe x16 slots (though they're likely PCIe revision 1.) However, your system may come with a Core 2 Duo. I would want to make sure that any XPS 710 you get has a quad core in it, or at least upgrade to a quad once you have the system. I think a Core 2 Quad would work acceptably well with an RX460, though I wouldn't recommend trying to run an RX 470 or better with such a weak CPU.
If you can manage a newer system for the same money, I'd go for that. Many RX 460s don't even need a 6-pin power connector from your PSU and can run on a 300w PSU.