You can choose whatever amount you want taking into consideration how much memory you have. If you have 32 MB of RAM, a 4 MB limit should be fine. Unlike adjusting the swap file size, playing with the disk cache size is not easy. Keep in mind that 1 KB is actually 1024 bytes. Like all changes to Windows, you will have to reboot for the changes to take effect.
Automatic virus scanning software can really decrease performance.
Such programs are always scanning every file as you open it for viruses. This continual scanning is a drain on the system. A good virus scanner is recommended, but not one that is checking files continually. A few more notes. When you upgrade hardware and install the drivers, always remove the drivers of the old hardware if you remove it.