If your monitor has resolution higher than 1280 x 1024 (basically bigger than a 19" monitor) it would be better to go with 512MB of ram.
I would not waste your money on either of those cards though.
You would be better off with something like a HD4670 for $59.99 after $10.00 rebate.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102820
However if you got a 650 Watt PSU I would spend the cash and hunt for something even stronger. Like an Nvidia GTS250 or an ATI HD4850 or 70. Hold back a month if you have to in order to get the cash. These cards are worth every penny.