|They both suck. Get a real gaming card & as kuwese pointed out, get one that has GDDR3 memory, not GDDR2.|
For $40, you can't go wrong with this 9600 GSO w/512MB GDDR3. It will easily blow either of the two cards you mentioned right out of the water:
Also, if you compare two cards that are identical except for the RAM amount, the 1GB card is *usually* slower than 512MB card.