I lucked upon this driver page
http://www.helpdrivers.com/ingles/listado/panellis.asp?marca=Sony&perif=cd1 which includes something for the PCGA-CD51 on WIN98. I have no idea if it's good or not.
The Sony pcg-505TR is listed as a 300Mhz pentium from 1999. At that time Win98 (and hardware of the time) supported very few USB devices because the specifications were not yet complete. Your most probable chance for an external device is to try and use the PCMCIA slot if you can find (WIN98 FE drivers) compatable components.
Keep in mind that you are limited by speed, OS, 64 Mb memory, and 6.4 Gb hard drive. That was a balanced package. Even trying to upgrade the OS to WIN98SE will give reduced returns if it works.
If you must input large files, many people have said they get an adapter for the hard disk ($5 to $20) so they can slave the laptop drive on a desktop and copy the raw files to it. The drive is then reinstalled and run on the laptop. It's not elegant, but it's functional.
Jim