I thought I'd share my tips/techniques I've found for getting these old cameras powered up again, as original batteries are hard to come by, and what memory cards I've found to work. This covers both Kodaks and Nikons but I included it in the Nikon section as they're more popular:
*****These are mods and involve soldering and playing with batteries. Be careful.***********
Kodak DCS 620X (Could probably also be used with all the 6xx and 7xx cameras)
Remove the original battery--new ones haven't been made in years and chargers are rare as hen's teeth. Also, the charger had this quirk that if the battery's voltage was below a certain level it wouldn't charge. You'd end up having to pre-charge it with a 9 volt battery before it would charge--someone even offered a service for that on Ebay a few years ago. Use a Quantum QB2 battery with a DC3 cable. Straight on power up and use. You will always need to set the time at first power on, otherwise no problems with use.
Memory card: Compact flash with an adapter, use a 512MB card or smaller and format it in the camera. Take a few shots and if you pop the card in a reader and you can see Folder 01 and Folder 02 and the image files in Folder 01 then you're good. Edit the files with Kodak's Photo Desk.
Kodak NC-2000 (Could probably also be used with all the Kodak 4XX series, maybe 5XX)
Has an external charging port but it needs a battery in there as otherwise it will just buzz loudly and not power on. Remove the battery, note where it plugs into on the motherboard. Cut the plug off and solder it to two longer wires, and plug it back into the motherboard. On the back of the camera is a molded button just below the SCSI port. Drill through that and route the wires through it. This will be a tight fit. From there you just need a 7.2 volt battery. I used the MS-D10 AA tray from my D700 as I'd never use AA's with it, and used molex (computer) connectors to make a junction to remove it.
Memory card: Compact flash with an adapter, I used a 32MB card formatted to FAT 12 with mkdosfs. If it gives an "E7" error this means it needs to be formatted. Sometimes also gives an "E2" error, just turn off and restart. This is because it was designed for a pcmcia hard drive and is expecting a "I'm powered up and ready" signal. Edit photos with Adobe Photoshop/Elements--for some reason Kodak's Photo Desk doesn't like the files, go figure.
Battery: Runs on AA's, just remember to not lose the battery tray. A Quantum QB2 battery can also be used with a DC5 cable, but the connector doesn't latch at all and can and will fall out of the camera.
Memory card: Works with a lot of compact flash or even SD cards in an adapter, probably the easiest classic to get running. I stick to a 1GB or smaller card to keep from having problems, and again format it in the camera. Edit files with Kodak Photo Desk.
Nikon DS-505/E2 (May not work on later models as they used a different battery.)
Battery: BP-D5 Ni-Cd. Split the casing with a knife around the seam (be careful not to cut the batteries). You can pull it apart at the top but it will stay glued together at the base. The batteries can simply be lifted out. The tabs on two batteries were the electrical contacts that two plungers pressed on through the two holes in top of the case. Note the +/- is molded into the battery case. Make some new metal tabs, solder wires onto them, and set them *on top* on the case and route the wires through the holes. Glue the tabs in place to keep them from moving and glue the seam back together. Drill a hole in the bottom of the battery case and route the wires outside. Afterwards same drill as the NC-2000, just need a 7.2 volt battery.
Memory card: Still working on that, as with editing files.
Batteries are still made for it, but it uses three. The main one is the NP-F550 or NP-510, just get a new one.
Memory card: Fun-fun. Will *only* work with a SanDisk 128 compact flash in an adapter, with one caveat. The camera has to be tethered to a computer and write a MDC.CTL file to the card, otherwise it won't work. The software is still out there but is Windows 95 era, I was able to get it to work on Windows 2000. Further complicating it is the camera's SCSI port is a weird "mini HD-50" connector that can be very hard to come by. Once you get all that straightened out it's simple, just use the card in a compact flash reader and make sure to never delete that one file. Edit the files with Picasa or Adobe Photoshop/Elements--the aspect ratio will have to be corrected either way.
Sony Mavica MVC 5000/7000
Batteries are still made as they're the same as Sony camcorders, just get name brand and not Ebay knock-offs.
Memory card: Nope, uses 2 inch "video floppies." Only source is Ebay and they're all old. You'll need to get a video floppy reader (such as the Panasonic AG-810) to even see the pictures. Images are stored as frame stills. I used a cheapy USB video capture device and Windows Media Player classic (free download) to view the images and then save them as bitmap screen captures, to edit later.
That's the big stuff I've gotten working.