In my opinion, I'd just get the XFX regular version. The coolers may be different, but they're all basically the same. I haven't heard anything about not being able to ajust the voltages, and the only thing reported in the user reviews (not a great source of info, but that would have come up) said that they hadn't done it yet, so the OC results were low.
I'd go with the XFX. Can't beat a low price from a high quality company, not to mention an amazing warranty.
That's what I do as well, but I get a little higher OC than that. I believe if you need more than that, you can change some settings outside of Overdrive to get more, but I haven't found it necessary yet.