This may seem like a stupid question, But I'm just making sure.
At the moment I'm running x2 IDE Hard Disks from a Sata Compatible PSU. Later this year I'm planning to upgrade the motherboard etc etc. I'm also installing a 80GB Sata drive, Will i still be able to run all three drives? or only two?
Yes is the short answer. To use multiple SATA drives you just need enough connectors for both data and power.
Most modern mobos will have more than two SATA port pinouts, so I don't expect any trouble there. Many PSU's have more than two power connectors for SATA units, but some do not. However, they still can supply enough power, so what you need is a way to convert the existing connectors. There should be two types of adapters available. One I know exists converts an unused Molex connector to one or two SATA power connectors. I expect there also is an adapter to convert one SATA power plug to two.
You did not mention optical drives so I ignored that, but Retired Chief is right. You COULD specifically buy a new mobo with TWO IDE ports for 4 devices. Or, you could buy a one-port mobo and get a SATA optical drive to allow using both older IDE HDD's.
You can also buy a JMB363 controller for around $15, which offers 2 additional SATA ports and one additional PATA connector as well, so even with a modern motherboard which only have one PATA connector, this would allow you to connect up to 4 PATA devices. Its a card with PCIe x1 interface, pic:
Note this example has its SATA connectors located externally, but most of these controllers have them internal as i prefer.