As far as RAM, it'd be helpful to know what motherboard you're using, and what RAM you're currently using. As pineapple says, depending on what you're upgrading from, it may not be worthwhile.
I believe 3 tiers up would be a 5870, which is a little more reasonable, but it's still expensive.
For upgrading your graphics card:
* what is your monitor's resolution? -- see the linked article for best recommendations at your resolution
* what is your budget? -- see if the recommendation matches your budget
* what games do you play? -- if you don't play FPS games, you may not need something as beefy as the article recommends