My iMac came with a 250GB (actual capacity is only 232 GB) hard drive and at the moment I have 200 GB free space. Those 32 GB hold currently all of the apple included software, 350 songs on iTunes (Plus the 350 copies that I have yet to delete when I transfered them onto the Mac, so a total of about 700 files) , several videos (about 4 GB worth), a DVD project, some images, and a boatload of software I have added since I got it.
So a 60GB HD would have an actual capacity of around 55GB, the 80GB drive would actually hold ~74GB. If I had the 60GB model, I would still have over 20GB of free space. If you have more than 20GB worth of documents, you must have some sore fingers and eyes!
That being said, if you opt for a bigger drive now, you can upgrade the RAM at a later date. Just pop a new stick in. Upgrading the HD would require you to backup all of the files from your old HD, get the new HD installed and loaded with OS X and then transfer the backups back onto it.
It would really be up to you. Do you think you will need the extra hard drive space? If so, I would get that now because it is easier to add RAM later. If you think that 60GB is plenty, which what you described it use would be seems likely, then you should get the RAM which would benefit you now.