The Gainward is dual slot (I have one, admittedly it's cooler is sitting on my desk). Dont bother with an overclocked one, you can do it yourself(and much better), in about 30seconds. I'd get a 512mb one as some games want that much for the highest settings. (CoH, GRAW, DOOMIII)
You want this one,
http://www.scan.co.uk/Products/Produ...oductID=468678 completely silent, and kinda single slot, really going to depend on your case, the back heatsink is quite big. Otherwise, just get the cheapest single slot one.