What card would you suggest I consider for under $250?
Since I'm NVIDIA biased, GTX 660.
Would you suggest an XFX? I've seen a lot of positive reviews about this company.
I did like them. I'm not sure what they're doing now since they went AMD only.
What are some important specs to consider when getting a video card?
Well... none. Like Intel and AMD, you can't compare graphics cards against each other. However, if you want to compare within the same company: # of shaders and speed of shaders, in that order of priority. Don't bother getting anything more than 2GB on a card if there's a huge price difference between that and the upper end.
Also, is my PSU adequate if I add a video card?
As long as the 12V rail is supply at least 25A-30A, you should be fine.
If I were to get a video card and wanted to dabble in beginner/moderate overclocking what would you suggest I add and/or replace? I'm guessing a heatsink and a psu would be the minimum but this is my first build and I am a PC layman. anything else?
That's about it. A moderate overclock may not increase your power consumption appreciably though.