compatibility for a new cassette and chain My cassette and chain have worn so I need replacements, My cassette is a 10 speed 12-30 and it says on it shimano hyperglide cs 4600 - I have struggled to find this and I don't think they make it any more, but I don't know of one that will fit, and my second problem is how do I know what chain to get it to match my new cassette, what matters?

Ok thanks for the replies, I am not too bothered about the ratios a 12-28 or something would do me fine.
I did a bit of looking up on what you said and I found this http://wiggle.com/shimano-tiagra-hg500-10-speed-cassette-1  I don't know whether it would work, what do you think?
And any shimano 10 speed chain you say will work?
4600 is the previous Shimano Tiagra series (4700 is the current version).
As far as I’m aware all 10 speed cassettes have the same cog spacing, so you should  be able to use any 10 speed cassette and chain. More expensive cassettes are usually lighter and maybe stiffer which makes them shift ever so slightly better. They are a wear part, I wouldn’t spend too much money on them.
Note: The current Tiagra 4700 groupset has different pull ratios for the derailleur and shifter, but that doesn’t affect the cassette spacing.