Normally I would say there isn't much you could buy for the riding you describe that would make a noticeable improvement. But since your wheels are 7 years x ~3000 miles = 20K so they may not be far out from having some rim or spoke issues so it may not be a bad time to go wheel shopping.

For what you are doing a basic everyday 'trainer' wheelset is what you want. You could serve your needs for $300-400 or up to double that if you want something fancy. If you are not racing, hill climbing or competitively time trialing the law of diminishing returns is pretty steep.

I would start with looking at the aluminum clincher offerings from Boyd Wheels or November. These are semi-custom handbuilt wheels with a couple of spoke and hub choices to suit your needs and well regarded by other forum users. The spokes used are standard issue that any bike shop should have available if there is ever and issue unlike what is common on pre-built wheels like Mavic and Shimano.

One more thing, another forum member is selling these: http://www.bikeforums.net/showthread...beless-ready!). They would suit you well and it really is a great wheelset for the price if it is still available.