If you buy the 512, you have the option of running it with the incumbent stick, jacking the total up to 768Mb. But I doubt you'd need it unless you or she intend to run RAM heavy applications such as 3D graphics or Video packages. The usual RAM hogs are high end games and a 933Mhz isn't going to be up to those even if you hike the memory to the system max.
I run two PCs that started out with single 256Mb modules and both are now happy as Larry running two after similar upgrades to the one you're engaged in, so if it was me I'd stick with what you've got ordered at the moment.
I expect however there will be others suggesting you max out the RAM as it's somewhat subjective, but my opinion for what its worth is 2 x 256 will be fine.