If you are thinking about the 9800Pro, I would suggest the 128MB version. It's performance is virtually identical, and it is $100 cheaper.
There is only one or two games that can use more than 128MB of memory. The other reason for more than 128MB of memory would be is you are running high AA AND high video resolutions. Of course, you would not do that with the 5900U, as the performance would choke, bu the 9800Pro could do it.