Neither. The memory interface on those cards is low (128-bit). That's why they're so cheap. You'll get much better performance out of a card with a higher memory interface (256-bit+). The ram isn't as important, you can get away with a 512MB card, but if you really want the 1GB it's only $15 more.
I.E. EVGA GeForce 9800 GT - 512MB 256-bit DDR3 PCIe x16 2.0
$75 after mail in rebate
If you really want the 1GB of ram, go with this card: EVGA GeForce 9800 GT - 1GB 256-bit DDR3 PCIe x16 2.0
$90 after mail in rebate
Memory interface is much more important than the amount of ram. You'll notice much more difference between the two. ATI increases theirs mostly with GDDR5 ram topping out at 256-bit and NVIDIA stuck with GDDR3 which tops out at about 448-bit so far. According to some sources, GDDR5 is twice as fast but you also have to take into account the memory interface itself with an equivalent ATI card to the 9800 GT only running at 128-bit. The same is with the higher end cards, the high 5 series are 256-bit and the GTX 2 series is 448-bit.