Sounds like the RD limits a cassette above 28, so the lowest double crankset you can find will extend your low end. They don't have to be expensive. A less-than $50 crankset will serve you fine. The 50x34s are nice to help your climbing. An example:

http://www.niagaracycle.com/categori...e-taper-silver

You can extend your high end, or at least get it back with the drop from 52 to 50 in front, by replacing the 14X28 cassette with an 11X28. Again, this does not have to be expensive. Example:

http://www.niagaracycle.com/categori...1-28t-cassette

I personally don't worry about the high end. Learning to spin more efficiently takes care of a lot of that. But then I also have been known to coast when I get beyond my high gear. I understand that others wish to press for speed at all times. Extending the low end, however, makes a HUGE difference for climbing.