1. Probably. Depends on exactly which IDE controller you have and which adapter, but generally, most work in most cases. Note that such an adapter can't in itself work around BIOS or OS size limits.
2. Once again, depends on the system. Not every PCI controller will work in every older computer - Silicon Image controllers don't like old PCI bus standards, some newer Promise cards don't have Win9x drivers etc. If it is supported, it is theoretically faster because the additional PATA -> SATA conversion step adds latency. However you're not going to notice that on a really old system. You also *really* won't notice the difference in bus speed, unless you're comparing PIO3 to SATA300 (but I doubt you'd get the SATA300 adapter working in a system with PIO3...)
3. Depends on the BIOS, but generally: yes. Some more modern BIOSs see it as a regular HDD you can choose, with older ones choose "SCSI"
4. SATA is backwards compatible, so yes, you can use those drives. I don't think there are any SATA-3 controllers for regular PCI or older, but again, this is irrelevant with the sort of CPU you have.
5. Completely depends on the hardware and software you want to use it with.
Two examples I have:
1) Pentium 100 system with Intel AN430TX board (ATA-33) running DOS. Using an el-cheapo PATA-SATA converter with an 8GB Sandisk SATA SSD. Works fine, is vastly faster than HDD.
2) Pentium3-750E system with MSI MS-6168 board running Win2k. Using a Promise SATA-300 TX2 Plus PCI controller with a 64GB Intel X25E SLC SSD. Total overkill, both on the controller and the SLC SSD, but that's the point of this particular system. Note that boot time would be better with the same SSD via a PATA-SATA adapter, as that would only be a bit slower (slightly higher latencies and throughput bottlenecked to ATA-33), whereas the Promise controller adds about 8 seconds to boot time.
