The base hardware requirements for Vista are generally higher than for previous versions of Windows. In Vista's case with the fancy GUI stuff enabled the graphics card in particular should be fairly beefy. Apart from that a modern/high spec processor, lots of memory help and a fast/large hard disk help. I don't think that as time passes Vista will become faster on the same hardware of capable of being run on older hardware if that's what you mean.
I've just been installing XP Pro on an old PIII 500MHz, 384MB RAM, 20GB hard disk machine today and am surprised by how fast it seems to run. On the other hand I haven't yet installed all the add-ons such as anti-virus/malware etc. However I doubt that this PC could run Vista and yet it's more than enough to satisfy the computing needs of the owner.
I've just been installing XP Pro on an old PIII 500MHz, 384MB RAM, 20GB hard disk machine today and am surprised by how fast it seems to run. On the other hand I haven't yet installed all the add-ons such as anti-virus/malware etc. However I doubt that this PC could run Vista and yet it's more than enough to satisfy the computing needs of the owner.