Based on the speed of your processor, I'm guessing it's socket 478. That's not a bad thing really. While you won't be able to get newer dual core processors, you can find ones much faster than what you have, provided you can upgrade that much. I'm sure Sony has some information about the motherboard the computer is using. See how far you can upgrade the processor. It will probably require you to update the BIOS, if you haven't done so already.

As for Vista, I just went to a product demo last week. I'll admit Vista has some pretty awesome new features, such as the ability to use flash drives as virtual memory, but I would honestly hold out for a while. It's Microsoft. Bugs are bound to be apparent. Plus, the DRM is going to make things a living nightmare for audiophiles. If you like the look, you can always download the transformation pack that makes XP look like Vista. And it certainly won't require you to shell out a couple hundred dollars.