Sorry for the late reply, haven't had much free time.
@Ratsel13 - The program you linked to did the job. Thank you! I tried a couple resolutions at first, and the screen would be cut off at the edges. After experimenting a bit, the trick seems to be to set your desktop resolution to whatever res you want the game to run in. All seems to be fine now.
@LeddBate - Wow, lot of info to parse there. :) I know you should avoid Dell if you have a choice. But this whole system was a package deal, which I got at a discount at the time. It was a big leap forward from my previous PC, and since I'm not very, how to say, financially flexible... I went ahead and picked it up.
After playing SE a bit more, I have say that you're correct; my current card is the weakest link. I'm not getting the performance at the highest settings that I thought I was. My FPS on Ultra settings in some areas will dip down to teens and 20's. High settings are somewhat better, but still not where I'd like to be. So... I'll probably experiment with individual settings and see what I get. SE seems a good deal more demanding, as it should be; Godrays and such. If I can't get good performance, I'll consider the GTX 1060.
But to answer your other questions - OS is Win7 Ultimate, SSD is 250GB, and the PSU is a Dell brand, so who knows how good it is. I planned on putting a solid 650 watt PSU from my old PC in this one at the time, but of course... space constraints. So it's either brute-force modification of the case, or spending the cash for a solid PSU that's the same size. I suppose it's worth mentioning that I got this PC used, so I assume that the PSU has been in there since the machine was built. I've had no trouble out of it yet, but I'm certainly no expert in this area.
In any case, I really do appreciate all the input and help. It's a lot to think about. Decisons, decisions.