For that kind of load, where you're doing mostly read but writes need to be quick, your file-system choice will probably matter more than your RAID choice. That said, the Synology DS1010 doesn't offer much choice in that respect. EXT3 seems to be your only choice, and it's OK at handling the "lots of small files" problem.
RAID5 gives you the most bang for your buck, but of course costs the most when writing. However, it sounds like writing will be a small minority of your IO operations. Read speed is what you need, and for that RAID5 is just fine for what you're looking for.
Unless this the kind of document management system where lots of docs get uploaded, but very few are ever looked at again, at which point your primary I/O type is writes. In that case, depending on the actual performance you get in the device, RAID10 is possibly more suited. Testing will tell which is what you want.