Your Maxtrash hard drive sounds like it's nearing death. Backing it up and replacing it would be the best course of action.
With Ultra ATA, the transfer rate will match that of the slowest drive, so your ATA/100 drive would clock itself down to ATA/66 speeds to match your old drive.
Caches speed up disk accesses, especially for non-sequential writes. The caches are under the complete control of the hard disk's microcontroller. The host controller can issue a command to disable caches in certain scenarios (such as when used as a removable disk), but that's about it - the computer's host controller doesn't interact directly with the cache and therefore doesn't really have anything to do with recognizing or using caches.
Another aspect to consider is the drive's transfer rates. Usually, only the fastest hard drives have transfer rates that can saturate a SATA/150 interface, let alone a SATA/300. The enterprise-class 15,000 RPM Seagate Cheetah 15k.6, for example, has a sustained media transfer rate of 164 MB/sec, where SATA/300 can handle 300MB/sec. (the Cheetah is used only as an example; it doesn't use SATA). As you can see, with a consumer 7200RPM drive such as the 1TB Seagate Barracuda 7200.11 (with its 105MB/sec transfer rate), SATA/150 would be more than adequate.
As for the PCI transfer speed, standard PCI (32-bit, 33MHz) maxes out at 133 MB/sec. 64-bit 33MHz PCI caps out at 266MB/sec.
If everything worked perfectly, unless you went with the enterprise-class drives, your bottleneck would remain the hard drive, at 105MB/sec with the Barracuda example.
I don't know about booting from an add-in SATA card in a Mac, you'd have to check the manufacturer's website for that. I think Sonnet makes an SATA card...
With Ultra ATA, the transfer rate will match that of the slowest drive, so your ATA/100 drive would clock itself down to ATA/66 speeds to match your old drive.
Caches speed up disk accesses, especially for non-sequential writes. The caches are under the complete control of the hard disk's microcontroller. The host controller can issue a command to disable caches in certain scenarios (such as when used as a removable disk), but that's about it - the computer's host controller doesn't interact directly with the cache and therefore doesn't really have anything to do with recognizing or using caches.
Another aspect to consider is the drive's transfer rates. Usually, only the fastest hard drives have transfer rates that can saturate a SATA/150 interface, let alone a SATA/300. The enterprise-class 15,000 RPM Seagate Cheetah 15k.6, for example, has a sustained media transfer rate of 164 MB/sec, where SATA/300 can handle 300MB/sec. (the Cheetah is used only as an example; it doesn't use SATA). As you can see, with a consumer 7200RPM drive such as the 1TB Seagate Barracuda 7200.11 (with its 105MB/sec transfer rate), SATA/150 would be more than adequate.
As for the PCI transfer speed, standard PCI (32-bit, 33MHz) maxes out at 133 MB/sec. 64-bit 33MHz PCI caps out at 266MB/sec.
If everything worked perfectly, unless you went with the enterprise-class drives, your bottleneck would remain the hard drive, at 105MB/sec with the Barracuda example.
I don't know about booting from an add-in SATA card in a Mac, you'd have to check the manufacturer's website for that. I think Sonnet makes an SATA card...