Everyone has summed it up well, Vista WILL NOT run efficiently with 512mb. 1gb really is the minimum, but if you can, upgrade & get more later. The processor should have at least 1.8ghz or higher, 4mb cache would also be good, but thats not really important.
As rbd said, Vista still needs work, so if you want to upgrade, you should still wait. The only good thing I see out of Vista is the fact it supports Dx10.....
Good Luck
