Welcome!
There is not a lot you can do. That model maxes at 1GB RAM (random access memory) and you have 75 percent of that now. Adding the extra 256MB to get to 1GB would not result in a detectable speed improvement.
Most of these older Mac actually run quite well for word processing and spreadsheets but really stumble with web performance. There are few "simple" web sites any more. Most use web technologies that have left all the pre-Intel Macs behind.
Some benefit can be gained from doing a few simple checks:
1) highlight your hard drive icon on the Desktop and then type the keyboard combo command i. That will produce this:
The bits I've outlined in red show how much of your hard drive is filled. You want "Used" to be less that 90% of "Capacity"--less is better here. The more room on your hard drive, the easier the system can deal with its data-handling routines. If the drive is too full, look for things that are unneeded or that can be archived to an external drive.
2) Run the maintenance scripts. Mac OSX features self-running maintenance routines that keep files organized and clean up unneeded files and caches. However, before OS 10.5, they did not run unless the computer was turn on and NOT sleeping in the wee hours of the morning. Not very realistic--the was fixed with 10.4 and later version. As you have 10.4, you will need to run the scripts manually a couple of times a month. Please refer to this excellent discussion of the scripts and manually running them;
Running the Mac OS X maintenance scripts
These may help a little but nothing can make your lovely G4 iMac run lie a newer model.