First of all ; the link doesn't work.
The warrenty will be voided when you put in a new card.
The powersupply ( i believe is 460W ) ain't powerfull enough for a mid/high end card.
You can go for a 6670 but that won't be that much of an upgrade. You could try a 5770 if you can find one. Also a GTX460, but it will be hard work for the psu.
Maybe the best and safest thing to do will be to upgrade your psu, maybe a 500W and a GTX560Ti or a HD6870.
What's your budget btw ?
O, and the 6570 probably hasn't got 2G but uses your mobo memory for that.