You can allocate more memory and specifically assign address ranges by doing so. Unless you are running Linux on your home system I wouldn't bother with it. If you are running windows it's going to optimize it anyway.
If you are using windows 98 download some "SMALL" memory management programs that are NOT TSR's. The memory leaks in 98 can be a little nasty. If you are trying to get everything out of the system you can for high frame rates, or whatever you are after, a memory cleaner and optimizer would be a great tool for you.
For more optimization kill your config.sys and autoexec.bat. Use a basic color desktop with no sounds. Increase your performance settings to File Server. And FULLY reduce your CD-ROM buffer settings.
That should be some good tweaking on your system and max out your memory and make it great for gaming or rendering.