replacing cassette & chainset I think the time has come to replace my rear cassette, it has begun skipping in a certain gear. I'd also like to give myself a bigger granny gear (something like a 28 or 30 would do). I haven't done much in the way of maintenance/buying new parts before so am after some advice.
Bike - Bianchi ML3 Alloy Reparto Corse.
The current cassette is a 9 speed.
Rear derailleur Shimano 105
Front derailleur Tiagra.
Chainset is 105 (looks like 53/39 which I think I'm happy with).
Shifters 105 brake-mounted.

So I guess chain will need changing as well, and possibly chainset (it has done a reasonable number of miles).
My question is - when I'm looking at cassettes I see there are a few 9 speed options, eg HG 50, PG 950/970 SRAM, Ultegra CS6500. I'm happy to pay a bit more for something decent. I guess what I have now is SRAM, but I don't know for sure! I very much doubt it's Ultegra. If I did go for the Ultegra would that mean I have to buy an Ultegra chainset too? And what about the derailleurs &amp; shifters, would they need to be changed to Ultegra as well? I don't know what combinations 'just work'. And what about Dura Ace etc?
I am looking at 9 speed because I seem to remember someone saying to me that 9 speed is the most my frame will accommodate, I don't know whether that is right. I can't find any frame specs that tell me that information which is really annoying (maybe I'm looking in the wrong place).
Thanks for the help, apologies for the noob questions.
Guy
Generally speaking, a frame/wheel hub that is fitted with a 8/9/10 speed cassette can take any of 8/9/10 speed because they're all the same width, just pack an extra cog or two in by thinning down the chain and inter-cog spacing.
11 may be possible but it depends on how the cassette can overhang inboard of the freehub without grinding.  12/13/14 speed is right out, unless you're buying a new bike.
The reason for staying with 9 is that you don't need to change the shifter and possibly rear derailleur.  The cost of these parts adds up, so going to 10 speed gives you only one more cog but costs a lot more.   Minimal change from maximal expense.

You're right the chain absolutely needs changing.  The chainrings may be adequate as-is, but its wear-dependent.  Also check your derailleur's jockey wheels - there its sideways slop more than tooth wear.

Mostly all 9 speed stuff is interchangeable. At 10 speed the standards start diverging, with differences in MTB/road, and tiagra doing its own weird mixed up standard.
Personally I'd replace with a 9 speed chain and cassette, check for wear in jockey wheels and chainset, and then keep riding.     Shout yourself some new bartape too, and the bike will feel awesome.