System memory = depends on the manufacturer's warranty. Usually you
can without voiding it, but you will need to check with Dell first.
Video memory = no. VRAM is soldered to the card and cannot be upgraded without upgrading the card.
Don't worry too much about the video memory if you just want the system for gaming. Worry more about which video card you're getting, as for gaming, clock speeds and cores and shaders are more important. If you're going to be doing professional rendering/design work, such as CAD, then go for more video memory.