replacing chain ring
 

i just got a single speed bianchi san jose. http://www.bianchiusa.com/08_san_jose.html

it has a 42t ring up front and a 17t cog in the back...i live in a very hilly area and i wanted to switch out the ring up front. Can i buy this ring and have it fit my new bike?
http://www.rei.com/product/698240

in other words what bcd do i have to get when i buy a chain ring?

You have to make sure the BCD of your crank is the same as the ring. I think the XD is 110mm and the ring you link is as well, but I would double check. It can be easier and more convenient to change the rear cog or freewheel. I keep several different sized cogs and freewheels around. They are fairly easy to swap around to get a different gear. Play with a gear calculator to compare different ratios.

You can get a smaller chainring but the smallest you can go will be determined by the BCD (bolt circle diameter) of your crank. I'm not sure about the Suguino crank on your bike but if it has a BCD of 130 mm then 38T is the smallest that will fit. If it has a BCD of 110 mm you can fit a 34T ring.

BTW,It would be a lot cheaper to get a larger rear cog. A 19T cog would give the same gearing as reducing the 42T chainring to a 38T and a 21T cog would be the same as going to a 34T chainring.

if I get a bigger cog...do I have to replace the chain?

It depends on how much bigger and how much room you have in the track ends. For each tooth difference you will need about 3mm of movement of the axle in the track end.

OK, I've done this a zillion times.

I'm pretty sure the bcd on your crankset is 110mm. Thus the cranks you have work with the chainring you wish to purchase.

In my opinion, it's easier to swap out chainrings than cogs. No need for special tools, and no risk of damaging threads on the hub. An inexpensive chainring can be found for $20. The beauty of singlespeed is that you can use lots of cheap bmx parts.

If you are going from 42t rings to a 39t ring, I'm pretty sure you will need to shorten your chain.

The beauty of running a bigger cog (and leaving the chain ring as is) is that it will also make the bike's drive smoother.

Having a few ss freewheels and the proper freewheel tool makes swapping things pretty easy as does having a few chain rings... neither is very hard to do and you might be able to run one chain with two freewheels if their differential isn't that great. 3 teeth seems to be the norm for maximum difference before the chain needs to be lengthened / shortened)

For a while I had a 42:14 on my Carlton which was good for everyday riding and also had a 52 tooth crank and spider that I could swap in if I was feeling like going insanely fast... that warranted another chain.

Now I just run a 52:18 on both my road bikes which is almost the same gearing as a 42:14 but is so much smoother.

You are right about the smoothness issue. I've recently been trending upward with regard to cog and ring sizes.

Like others have said, it looks like you have a 110bcd crank. Here's a chart you can use to confirm the bcd by measuring between two adjacent bolts: http://sheldonbrown.com/gloss_bo-z.html#bcd

If your hills are long, then you might want to switch to a freewheel (with brakes) if you aren't running one already. If you're gearing down for the uphills, you'll spin out that much sooner on the descents. The first real climb I did on a fixed gear was about six miles, and the long descent was way harder then going up.