I personally avoid Vista whenever possible, but let's be honest. XP was a greedy system. It required 1GB RAM, while the other OS were working fine with one quarter of this amount. It took about 5 GB from the harddrive, while e.g. Win98 something about 0.5 GB (with virtual memory file). And it used to have compatibility issues too.
XP SP2 took 1.3GB of HDD space installed. I don't have clean drive numbers on SP3.
Further, my father is running XP SP3 on a system with only 256MB of RAM. It's certainly slower than it used to be, and he would benefit from 512MB+, but the commit charge generally stays around 300-350MB. Which is a bit more than the RAM, certainly, but it's hardly 1GB.
I myself ran SP2 on a system with only 128MB of RAM (yes, you heard me right) for a little while, but rapidly discovered that it was insufficient.
As for XP's "requirements", it won't install with less than 64MB of RAM.