I'll assume you're going for this one:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-087-HS
Card brands are a very interesting factor, for instance. When the 7970 and 7950 started getting non-reference designs, some manufacturers were using 7970 PCD's for 7950's which had a few more layers to it which made it a little better for overclocking.
HIS are a very good brand, well known to 'teh internehts' as being inherently good, with some failures of parts, that is to be expected though. I imagine this card will be fairly good, I'm 90% sure there is an Asus DirectCUII and a Gigabyte versions of this card with bigger, quieter coolers (Probably not that much quieter though). However I think these versions are about £10 - £40 more expensive (If I remember correctly), probably worth stretching for if £10 - £20 more, but not if it's any more than that.
Core clock speeds are a reasonably bad way to judge a card unless they are using the same chip (Pictairn, Tahiti etc). HWCompare will come to the rescue though to get a rough judgement of how two cards may perform against each other, your situation specified here:
http://www.hwcompare.com/15671/geforce-gtx-660-vs-radeon-r9-270x/.
I'm not sure how accurate these are though as they are theoretical, but as you can see, in some areas there are major differences. You can usually judge how much more performance you'll get from the core clock (ie, the standard core clock for a 660 is 980Mhz) and if you were to overclock it to about 1100Mhz you'd probably see about a 10% improvement in performance (Though that is never usually the case, it's usually between 5 and 7% as there are many other factors that can't be changed).
My suggestion would be to go for the R9 270x you specified, I've just seen the prices for the Asus and Gigabyte one's as £30 more. My bad
