Will upgrading RAM on my Macbook Pro significantly improve speed?
It might be different depending on what type of app you are using, but generally speaking, it sure is beneficial to update from 4 GB to 8 GB since 4 GB is not a lot for a Mac and you are using Xcode.

The iPhoto app might not change much, because it has to load lots of photos, its latency depends more on the disk instead of the memory.

The value of virtual memory might not be very informative, you should look more at swap used right below virtual memory, this is the memory that is actually stored on the disk since the physical memory is not enough.

If you see that swap used is often more than zero, your priority should be absolutely to change for 8 GB memory. Otherwise, installing a SSD might be a better choice though that's more expensive...

edit 1:

Generally speaking, I would recommend upgrading the memory over upgrading to a SSD.

A SSD affects only the loading speed of things, that is, the time that it takes for the computer starts up or for an app to load itself. But the size of memory affects the speed of apps when you are opening lots of them. 

On the Mac, the concept is switching more and more to holding the app inactive in the background instead of closing them completely, that enables the apps to respond quickly since we don't have to load them again from disk. 

Despite Apple's memory compression technology, opening lots of apps still takes a considerable amount of memory, so upgrading the memory prevents the mac to swap some memory to disk, and makes the mac faster when you have lots of apps in the background.