True - but consider that you have to use one of those slots for video (vs. AGP video slot in QS), another one to add USB/Firewire, and yet another if you want to use ATA hard drives and optical drives.
Absolutely.
- Upgradable processor up to dual 2GHz G4 - starter systems around single 800MHz G4
- 133MHz system and RAM bus vs 50MHz. Also, PC-133 RAM is much cheaper and easier to find.
- Faster ATA drives vs more expensive SCSI drives or buying an ATA PCI card for the PT
- AGP video = faster and better video cards, including native support for Quartz Extreme and CoreVideo
- Built in Firewire and USB1.1 (PCI card required for USB2)
- OS X up to 10.5.x - and OS 9
Yes, but not easily or well. Google for XPostFacto - the utility you need to use to install OS X on unsupported machines. IIRC, it will only work up to OS X 10.2.x
Really, compared to doing any upgrades to the PT, you could find a base level QS for the same price which would be a much more powerful machine out of the box - assuming the market for used Macs near you is anything like near me, or the USA.