If you are updating it a LOT then the files contents being stale is not likely to be an issue for you. If so, stick it on tmpfs, truncate the file on update and rewrite into it. That would be the cheapest method as it is not likely to use disk at all.
The next closest is to truncate/write onto a filesystem that has the noatime mount option set and journalling turned off. But again its risky if you crash out as you might possibly lose the data.
After that its noatime again with journalling turned on.
Remember, linux buffers writes and syncs to disk at determined intervals so you wont normally 'feel' the impact of a write from an I/O standpoint (unless its very heavy writing but thats tunable). You can alter the conditions to sync to disk too so you can have the write buffer fill up for quie a long time before syncing to disk.
If your after doing something really clever.. use fallocate to preallocate space for the file which would be its maximum possible value. Then, mmap open the file to read it directly to memory.
Then rewriting will be near instantaneous (but lossy if you had power loss). You can then control when to flush back to the disk with the msync call.