In reply to VS4b: The Planet X Model B get rave reviews for value for money pretty much anywhere you look. They will certainly be the lightest off the shelf wheels up to £200.
The quality won't be amazing, but certainly no worse than anything else at the price and they won't be the strongest. Nor do they have the big brand kudos.
If you're not a brand snob and not a heavy/powerful rider, to me it's a no brainer.
Oh, and as I'll always say, you'll probably notice more of a performance benefit for changing the tyres and tubes first. Save yourself some cash, too.
Rob