AmigaOS is extremely efficient in regards of the amount of memory and computing time required. With only 1 or 2 MB RAM, you can already multitask several full-blown application programs without any noticeable delays. And the AmigaOS and typical applications need relatively little hard drive space as well!
Depending on the processor variant, you can expand the RAM up to 10 MB or theoretically up to 2 GB. As soon as it's present you can use it directly, there is no need for additional memory management software. The limit of 2 GB is valid for the Amiga models A3000 and A4000. On the mainboard of these computers you can plug in up to 18 MB RAM, and for every of the four Zorro-III bus expansion slots you can today buy cards with 256 MB capacity. So there is already the possibility to expand the Amiga RAM to over 1 GB!
Yes. All current models have a harddisk built in or have provisions to get one built in internally.
So fast that a well-known harddrive manufacturer uses Amiga 3000's to test its SCSI harddrives, because this model loves fast drives and can exploit them to their maximum.