The cheapest thing to get off the bat would be more ram. Just make sure it's the correct one for your mobo. 4 gigs in the lower denominator, 8 for the higher.
Next would be a video card, but depends on how much you're willing to pay. I would go with the 8800 GT, but you could also go for one of the X60 series, such as the 360, 460 or 560.
At the same time however, if you're more into rendering stuff, then you might want to look into a CPU instead, and a Quad'er at that.
What exactly do you usually do more time then not? Mudbox or UDK? Do you render alot? Also, what price tag is your budget in?