If this is for the machine listed in your system specs then the amount of processes and RAM is not going to to cause system slowdown. There are more important factors such as CPU usage, I/O Reads and I/O Writes.
ESET NOD32 has often been considered very light over the years and it uses 40-50MB. Some find Microsoft Security Essentials heavy but most don't. It uses 50-60MB or sometimes more.
Avast! and Norton are two of ones that use the least amount of RAM. I don't notice either of them on my low specced machine. The last time I used Avast! it tended to use a few MB more when surfing with Firefox but on average they use about the same. Norton currently running: