if you have a 5yr old motherboard, perhaps the answer is to buy a new motherboard (or new computer?). I realize that's easy to say and not so easy to afford, but at some point trying to upgrade these things is more trouble than its worth..Argh, I think you're right.
I agree. If you're going to buy a new disk, I would indeed get a SATA drive. But in addition to that, I would replace my motherboard with one which had a SATA interface as well as a PCI Express graphics interface. That means buying a new PCI Express graphics adaptor that's compatible with Vista. You can still keep Windows XP, although if you plan to upgrade to Vista, I would do so when you replace the disk drive.
The only thing that is important and lacking from my system right now is a robust backup method, which is why I'm splurging on the two 500GB hard drives.
Now that's a whole different issue. If you're looking for a backup disk, I would buy an external drive with an "eSATA" interface. This will have a little different connector than the standard "internal" SATA connector to facilitate better shielding for longer cable length.
When buying an external drive, if you're a computer professional, I wouldn't fool around. Get something like the G-Drive, which has everything: Aluminum case, no fan, eSATA, 400 & 800MHz Firewire, and USB 2.0. Of course, the eSATA connection will be the fastest followed by the Firewire connections. Here's an example source: http://www.academicsuperstore.com/market/marketdisp.html?PartNo=752728
If you're not a computer professional, getting an external drive with only an eSATA and USB interface would be good enough. If it's a high performance disk (e.g. it gets hot), I would still shoot for the aluminum case w/o the internal fan. Slow (cool) drives can be put in plastic cases.
The G-Drive I mentioned above allows air to circulate both above and below it so it's cooled on both sides. That's what you want to shop for if you're buying a high performace (runs hot) drive. Most people may be happier with a slower, cooler-running drive though.