Join Date: Aug 2003
Bikes: '03 Fuji Roubaix Pro; '03 KleinGi Attitude; '06 Soma Rush; '04 Surly Cross-Check; '06 Soma Rush; '07 Scott CR1 / Chorus
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I don't know anything about your current crankset, so no idea if there's a way to fix it...
Cranksets don't, for the most part, care what else is in your drivetrain. A crankset sold as part of a 10-speed group will work fine with your 9-speed stuff.
Your shifters determine how many cogs you can have on the cassette. If you have 9-speed shifters, you need a 9-speed cassette; you can't just put a 10-speed cassette on. The spacings are different, and the 9-speed shifters won't shift a 10-speed cassette. That being said, you can still easily buy 9-speed cassettes and chains. Your local shops should be able to order them in, or you can get them at many online stores (e.g. performance, nashbar). In any case, it doesn't sound like you actually need a new cassette.
A 50/36 crankset will be a compact crankset - the diameter of the bolts that attach the chainrings is smaller, hence you can put smaller chainrings on. A compact crankset gives slightly lower overall gearing, which is great for climbs and arguably more practical gearing for a recreational rider no matter what the terrain. Not many of us have the power to use the tallest gear on a standard double drivetrain on anything but fast downhills.
Do a search of the forum for "compact crankset", and you'll find a heckuva lot of threads debating the pro/cons.