I would say get a better graphics card, you can always add RAM as you can afford it and 2Gigs should be plenty for almost all tasks.
I have had both situations and to tell you the truth I very rarely use 4 gigs of RAM but if you get a good graphics card with say 256 of inbuilt Ram and good Open GL support you will probably find you won't need much more RAM than 2Gigs.
Just my opinion of course.