I think you've got to decide what you're willing to spend then try them out if you can - people here will help narrow things down a bit - I'm in the same boat right now with the added problem of being in Australia. A lot of the retailers I've spoken to here have tended to talk too much waffle on this subject to be honest with more than enough of them playing down the impact of cables.
This argument has I'm afraid been lost on me as last time I had a decent HiFi - about 10 years ago I was absolutely shocked at how much better it sounded with improved interconnects and speaker cables. Back then I was lucky as the retailer (in Middlesbrough) sold me the rest of my system and made excellent recommendations for me at various price points and was right on the money so to speak.
If only I could find a retailer here who had similar experience.... For the record, I've spent about A$6k on my system so far and am prepared to pay perhaps A$500-600 on speaker cables (3 sets, 1 biwire, 2 x bi-amp). If I had more expensive kit I'd go a touch further as I'd expect to reap the benefits but in this case I think beyond that, given the quality of the rest of the system, its probably getting into the area of diminishing returns. Not sure if that helps?