Branford Bike: Campagnolo 10 Speed MK 2 Cassettes. Dave Malcolm and I are going to ride the "STP":http://cascade.org and he asked me about what cassette in the rear to use. He has an 11-23 cassette and is wondering if he should get a bigger one. Here are some facts, but
Well. I'd reco a 12-25 in the back. A 13-26 doesn't add much at the top end (a ratio thing, it is just 26/25ths longer). If you want "Excel":http://excelsports can get you a pretty good price. About $150 or so as I recall the the record titanium/steel cassette. If you can wait, "Sdeals":http://sdeals.com or "Total Cycling":http://totalcycling.com has better prices (dependent on exchange rate, they are in the uk). Also, "Ebay":http://search.ebay.com/search/search.dll?GetResult&st=2&SortProperty=MetaEndSort&query=campagnolo&ht=1&currdisp=2&itemtimedisp=1&category5=42332&combine=y&from=R9 has auctions on these, but you have to know where to look and get it from a reputable seller.
You actually can't go beyond 13-26 unless you replace just about your entire drive train. The derailleur you have is what is called a short cage so only supports up to 26 in the back. Also you would need a new front derailleur. About $400 plus, so that isn't super practical.
By the way, last piece of advice is to make sure you follow the shifting recommendations closely. The campy drive chain is very delicate racing oriented. If you run improperly even a short time, you can trash the rear cog, the chain and the front cog. The chain should last 2-3,000 miles and a cassette up to 7,000 miles if you take care of them.
What's proper? Make sure when you shift, you never, never run the small ring in the front with the smallest three gears in the back. Also the large front ring should never run with the largest 3 cogs in the back. I destroyed my drive train in 500 miles doing this.
Voice of experience. We should also check your chain length as well campy chains are notoriuous for having relatiely short life. Like 1500 miles if you aren't careful. Given how hard this bike was raced it could be worn. That's is lengthened so that it actually wears the cogs.