I can't tell you the exact memory usage of McAfee Antivirus, but I have the software package here at home and the mem usage is much lower than that of Norton IMO, it runs very smoothly, even when I used to have 512Mb RAM.
Considering how new the software is, the minimum specs are quite good:
CD-ROM Drive
Microsoft Windows XP, 2000, Pro, ME, 98
32MB RAM
65mb hdd space
Pentium 133MHz CPU