Check the load table and make sure it is rated honestly, i would expect at least a combined 12 V current of 65 A for a Dell 875W PSU, if it has significantly less available take that into account when you are looking at selecting a graphics card. A good PSU should last 5-7 years, it is one of the few things you should expect to move over to your next system so you should have no problem moving that over, i don't think anyone is using proprietary connectors on their PSUs anymore.