replacing a chainring
 

i have a 46/36 crank up front (cross bike) and 12-25 in the back...Is it possible to remove the 36 ring and replace it with a 34? Or would i be better off going with a 12-27 in the back...

You need to answer this question yourself. We're not going to be riding the bike you are using.

http://www.sheldonbrown.com/gears/

Yes, it is possible to replace your 36t ring with a 34. They are available, will fit your crank, and will work fine with your existing front derailleur.
A Shimano short-cage rear derailleur can handle a maximum cog size of 27t, so you could install that too if you want.

i just wasnt sure if the 12 tooth jump would be too much....i may go with a 34 and 12-27

Operator is on the right path. Knowing the gear inches would help you know how much the differences in gears would affect you. What would be better would be a chart of them all. Google for one - they are pretty common and I have one as a background on my other computer.

If you really need a lower gear, do both. A 36x25 produces a 39 gear-inch low gear and a 34x27 gives you a 34 gear-inch low gear so the difference is significant 13%.

Shimano's 12x25 and 12x27 cassettes differ only in the largest two cogs so you give up nothing in the smaller, more used, cog spacing.

You may also want to replace the chain so you get equal wear.