http://nz.answers.yahoo.com/question...65 959AA7GOzu
In reality I imagine he won't be seeing too much difference between the two. Generally though, faster memory outperforms more memory.
For Civ 5, 512mb of memory (and the fact that the card is 4800 series) falls right into the recommended requirements:
Recommended System Requirements
Operating System: Windows Vista SP2/ Windows 7
Processor: 1.8 GHz Quad Core CPU
Memory: 4 GB RAM
Hard Disk Space: 8 GB Free
Video: 512 MB ATI 4800 series or better, 512 MB nVidia 9800 series or better