Previous versions of Logic have been less processor-hungry than Garageband. I'm sure you're aware Logic has just been updated to version 7. It's quite a major upgrade and I haven't personally seen it yet, but early reports seem to suggest that it's still less hungry than Garageband. So yeah, you *should* be able to use more Instruments, AU, effects... and you can use Apple loops too.
It will do everything Garageband does and more, although perhaps not in exactly the same way - Logic isn't known for being the easiest software to get into. Although version 7 brings a new Apple-ized, cleaned-up interface which is apparently more, er... logical.