I am setting up a LAMP server, and am looking at using a spare 32bit box I have.
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no difference in stability. it theoretically won't have access to as much memory (but running it on 32 bit hardware means you don't have that much anyway) and slightly slower for programs optimised for 64 bit architectures, but a lamp stack will run fine.
i only just changed over to a 64 bit server for my lamp machine a couple of months ago. no real difference other than future expansion
from my experience since I did the same question you are doing I have the following:
Basically, in your case, use the 32Bit Server and enjoy a very good server indeed.