performance: large includes vs small includes with lots of reads

I'm trying to tweak my server a bit and wonder if it is better to have a large include file, say 20-40 kb with 15 user-defined functions, of which maybe 3 or 4 are used on any given page, or to have each function in it's own file and include (disk access/read) them only as needed. Is it faster/more efficient to read one large file or several smaller ones? It's a RS Linux server with a gig of memory. I'm also looking at PHP Accelerator and such. Do those store only in memory what the script needs, or does it include all that you require or include in, as well? Thanks, Hans