It depends on that app, if you're running a memory hogging very intensive application, and you have a 64 bit operating system, I'm sure more than 4 gigs would help. But for 99% of people, 4 gigs is going to be enough, and you can always upgrade later if you by 2x2GB sticks.
And a 32 bit OS can only recognize 4GB - the video card's VRAM. So if your video card has 512MB of VRAM, only 3.5gigs will be recognized if you install 4gigs of RAM.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.