if you are going to get a 128 bit card then 512MB of RAM is useless as the bit rate is unable to use all the RAM
256 bit cards are significantly faster and are perfectly suited for 512MB cards.
Like I said before, you will need to spend a bit more if you really want to see a performance increase from your current card.
(BTW the 512MB cards with 64-bit interface you suggested in the first post would be an absolute waste of money)