Memory is 25% of the "Overall Rating". Each 128MB will bump your memory rating by 33 points
On a cow that normally scores 30, adding 128MB will bump your overall score up by about 8 points, which is about 35 UD points per day. Adding 256MB would increase your overall score by about 16 for in increase of about 75 points per day.
You can figure it out yourself using these formulas:
Overall Performance = CPU x 40% + Memory x 25% + Storage x 5% + Network x 30%
Points = CPU time x Overall Performance / 5.4
32MB = 9
64MB =17
128MB = 33
256MB = 67
384MB = 100
512MB = 134