Currently using PATA, how much better is SATA?

There are some gains to be seen but not enough to justify buying new drives

For ordinary harddrives, there is not alot of difference between the latest PATA-133 (133MB/s) and SATA/150 or SATA/300 (300MB/s) interface. There is *some* difference, because even if the drive could hit only 100MB/s, the propagation delay of the signal sent through the cable is lower for SATA/300, which does lower latency a small bit. The same can be seen when using PCI-express versus PCI.

If you connect two harddrives to one cable, and use both at the same time, it would be slower ofcourse. But as stated above it doesn't warrant not using them anymore. Do note all 7200rpm disks do use a fair amount of power; about 8W when idling; while newer 1TB+ 5400rpm drives can do with less than half, so the watt/per/GB ratio of old drives is quite bad. That's why i stopped using my old drives, but instead used them as offline long-term backup solution.

That's a windows issue. As you continue to use the system and install/remove stuff, core system files fragment (which aren't defragmented because they are "unmovable files" ) and also multiply; the system registry and startup entries become more crowded so the system boots less fast. Re-installation becomes an option. So its not your disks which are slower, but your operating system is getting slower as you continue using it and expanding it; it becomes less clean/tidy/compact and thus more work for the disk. An SSD really helps here. Oh and two HDDs on one cable should be fine, unless you are using both at the same time ALOT, like in software RAID configuration.

As i'm using Ubuntu linux as a workstation OS, i do not suffer from this problem. Also i don't have to reboot very often and leave my systems on 24/7. A large amount of RAM will cache any programs and the HDD is only used for writing my firefox profile and system logs, etc.

The key to performance experience by the computer user is not in the hardware, but rather in the software design. Computers are already so fast that they could be made without any "quirky" or slow behavior on the interface/frontend. Even if the CPU is ramped at the max, the interface could still be fluent and responsive to user interactions. However, due to less-than-ideal to downward bad software design, you may get bad interaction even with extremely fast hardware. Threading and multi-tasking is key here, and the software world has to change its usual single threaded programming model in order to unlock the performance required by modern computer systems.