You can choose whatever amount you want taking into consideration how much memory you have. If you have 32MB of RAM, a 4MB limit should be fine. Unlike adjusting the swap file size, playing with the disk cache size is not easy. Keep in mind that 1KB 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. I recommend a good virus scanner, but not one that is always on and scanning. A few more things. When you upgrade hardware and install the drivers, always remove the drivers of the old hardware if you remove it.