His system should be able to handle way more than one or two FX on a couple tracks. Using a system with 4 gig RAM I can get 20-30 FX running without issue. I'd venture to guess something else is going on.
Shared IRQ? Could cause processor interruptions
Fragmented disk? Could cause read delays
Disks going to sleep? Can cause weird things to happen

If that particular plugin does seem to be the problem, see if there is a DXi/VSTi option (some Izotope plugins can be installed in either format and one may work better on a given system).

I have never seen a system where using ASIO allowed more FX to be used. By nature MME has more buffer time available, so is usually able to run 'more stuff' more smoothly. ASIO usually makes FX/Synths respond faster when real time response when needed, but MME should not introduce timing issues.

Also, closing down and restarting every so often frees up resources. It's a habit here, like saving often. Every time you do an action that can be undone, that action has to be stored somewhere temporarily. That can be anything from an edit to recording a new track. These add up over time, and closing down frees up those resources. I know some people that leave PT/RB running for days .. I do not do that, nor suggest it.
Also, recording to a second drive (separate from the playback drive) helps; it spreads out the load.

I do not work here, but the benefits are still awesome
Make your sound your own!