I would say get the cheapest AWG 14 OFC for any system. The (recently) oft quoted Van Damme pro cable at 2.5mm thickness will be 99% as good as anything else...imo. It isn't "cheap" (£3 a metre), but having used that and stuff costing 4.5 times as much I wouldn't hesitate to recommend it.
And although I am not quite in the "all cables sound the same" camp, I would say it is the LEAST cost effective upgrade to any system as the differences are subtle at best (so long as you are not upgrading from bell-wire).