With the Protect My Mac feature enabled, iAntiVirus sucks up huge amounts of CPU resources whenever I'm downloading, slowing down my MacBook Pro 2.4 GHz Core 2 Duo.
But ClamXav, guarding against the much huger list of Windoze viruses, does not cause a performance hit when used in the identical circumstances.
And MacScan does appear to offer an equivalent level of protection. So I'll stick with ClamXav and MacScan, and keep my MBP from bogging down.