Why do yo say that QV seems to be unaffected? How do you measure this? Do you use the Task manager and monitors the memory used while QV is proccessing?
The performance will be affected by the type and amount of data fields, and your script. Also, in final applications, it is strongly affected by the data model created.
So, if you don't need more memory for your apps, QV will not use more memory if you have more memory. If you need more memory that you have installed (or have a 32 bits SO, as Daniel said), the performance will not affected too, because the paging is used too.