New rear cogs needed for climbing

Your post lacked some crucial info. Assuming you have a road or touring bike and it is equipped with a later Shimano 9spd rear slide on cartridge style rather than the screw on cartridge, your options are pretty wide. Check out Sheldon Brown's gearing chart showing compatability of various Shimano gear combinations.
http://www.sheldonbrown.com/k7.html
My road bike is all Shimano 105 componentry but to handle the hills (and keep up with the young guns), I fitted a 12-27 Ultegra rear cluster. The chain and chainrings are Shimano 105 (39/53). It all works like a dream. I do not even have to fine tune the rear derailer when I change back to my training wheels which have a 12-25 Shimano 105 cluster.😉

Tezza,

How much better do you find the 27 tooth rear cog in the hills? I've got a 12-15 cassette at the moment and am considering getting a 12-27 (or 13-27 if there is one) to help me survive ultra steep hills.

Rek,
It really makes the difference on race day! I run the 12-25 on my training wheels and the 12-27 on my lighter race wheels. I can stay seated a lot longer (and as the experts tell me, I save a bit of energy by doing that). I do not sacrifice any high end gearing. The cogs on the two clusters are the same apart from the two lowest.
ie 12,13,14,15,17,19,21 are all common. Then it is 23 and 25 as opposed to 24 and 27. There is a bit of a jump from 21 to 24 but, sometimes that just feels soooo good. Also I like to keep my cadence between 85 and 100 and the Ultegra set helps in this regard. Even if your races cover only moderately hilly terrain, I'd recommend you try this combo out. I think you also save a poofteenth of a gram with the Ultegra cluster too.
Tezza.🙂

you can calculate the gear inches for any combo using this formula...

Gear inches = (# of chain ring teeth X wheel size) divided by
( # of cassette cog teeth)

if a 700c wheel, use 27 (that's the standard)

ex. (39 x 27)/25=42.12 gear inches

(39x27)/27=39 gear inches

(39x27)/23=45.78 gear inches

therefore the difference between a 25 and a 27 in the rear is close to the difference between the 23 and 25. will that be enough for you? if not, there are other games you can play. if you have, or install, a long cage rear derailleur you can use a mountain cassette with a 32 (maybe 34) tooth cog.