Creating an archive which you can make now, put away and have it be usable and searchable 50-100 years from now is basically a pipe-dream. It is going to require maintenance over the time to keep it working/readable.
First the reasoning for this: it is impossible to determine what file formats and hardware interfaces will be readable 10-15 years down the line, let alone 100. Do you have anything that can read the original 8" floppy disks? What about fortran punch cards?
Secondly the longevity of storage means is dubious. Hard Drives are not meant to be stored offline for a long period of time, and their online lifetime is generally considered to be ~5-7 years. DVDs and CDs also have issues with longevity and are not proven.
Third is the issue of having the programs to open the files. Will the software 50 years from now be able to open the files? will you be able to install the old software on the new computers?
Alright, pessimism aside there are some things you can do. Probably your best option is to create an online server dedicated to the storage of this archive. Build a computer (or even buying a small tower server from Dell or else where) with a mid-line CPU and pack it with say 4 1 TB hard-drives installed RAID-1 giving you 2 TBs of storage with mirrored redundancy.
Install, depending on level of expertise, something like Ubuntu server + webmin, or Windows home server and attach your new server via gigabit ethernet to your network. As you scan in the files, dump them on to the server.
Now, you will want another layer or redundancy, off-site storage. For this I would say get 2 TB worth of external HD and weekly, as you scan in the photos, backup the entire contents of the server drives on to the external HD and store them offsite, at your work, a safe deposit box, or a relatives. This provides a layer of protection against fire, theft etc. Note that once you are done with the project, you should spin up the drives ever couple of month and make sure everything is working.
The last step would be constantly maintaining the system. As new media and interfaces come around and it looks like what you are currently using is going by the wayside, you will need to upgrade and transfer all the data over.
Now, there is the issue of actually digitizing all those photographs.
Are you scanning negatives or prints? And in what quantities?
If you are scanning something like negatives in fairly large numbers, I would certainly recommend something like the Nikon Coolscan V which will be a lot faster than a flatbed scanner, but if you want to cut costs, you can use something like a Epson perfection 4490 (which I am currently using for a similar project) it is just slow, and technically lower quality, but for family images, usually we aren't talking about fine art quality anyways. Whatever you get, make sure it has Digital ICE it will save a lot of time in removing dust and scratches. It isn't perfect but does a really good job.
Now, as for the file format, I would say you are probably safe using something like tiff that has been around for quite awhile and is well supported. Since storage space has gotten so cheap now, I don't see any reason not to safe them as 16bit Tiffs. I would also, however, store a copy as a jpeg just in case. Now, once again, as time goes on, you or someone else might have to go through and swap formats if it looks like jpeg or something else is going to die out. I think it is unlikely that something as well used as jpeg is going to just up and disappear but it is something to be conscious of in the long term.
That is a start, for more detailed info it would help to know quantities etc and what your end hope is for the images.
As a final note, remember that your film will be archival and prints are always going to be able to be viewable, you should work to store them in a way in which they will last. Perhaps a fireproof safe would be in order as well 
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((just being funny!))
