If an individual app is significantly slower with lots of other apps running, I might suggest more ram.
If you are doing lots of demand paging, it will look like a slow cpu. Use the task manager/performance monitor to check the hard faults per second under load. If the sum of all the working sets is near the total of your ram, more ram will help.
In a similar vein, look at the cpu busy graph and see if both your tasks are consistently fully utilized.
If they are, changing to an 8 core will help.
If you have not yet done so, consider a SSD. It is usually the most dramatic performance improver you can buy.