At a pinch, even just a ram upgrade should be ok, as I said, you may or may not be able to run the Aero interface, as that requires a DX9 graphics card, I dont know if the VIA will have any/enough DX9 to do the job.
Vista will need around 8GB of disk space, plus some more for the swapfile. An 80Gb hard disk will actually give around 74GB of usable disk space, you'll probably have around 60GB or so free for applications and data. Its up to you to decide if this is enough for your needs.
Not everyone would agree, but I like XP 64bit edition, there are some issues with drivers sometimes, so you need to make sure you can get drivers for any hardware you have (printers/keyboards/mice etc). And XP64 is a lot lighter on memory requirements.
In the long run, XP64 bit is a dead end, its unlikely that many hardware manufacturers will develop 64bit XP drivers, they will simply do Vista drivers. However if they currently have the XP 64 bit drivers available then I would expect they will continue to release maintainance upgrades etc.
Another thing to remember, is there are going to be a dozen different versions of windows Vista. The 'lighter' versions will have less features, but also a smaller systems requirement to work efficiently. So it all depends on what you are planning to do with the computer.