What is the better upgrade?

I'm looking to either upgrade my GPU or get an SSD. The problem with my current GPU is it doesn't work properly, I need to have it underclocked so it doesn't crash. I'm looking at either the 5870 or 5850 right now, my resolution is 1920x1080.

5870 = $499 CAD

5850 = $349 CAD

SSD = Depends :)

Specs

i5 @ 3.6GHz

4GB G.SKILL DDR3

EVGA P55 FTW

XFX HD 4870

WD Caviar Black 640GB

Windows 7 X64

i would definately go for the graphics card over the SSD.. you will get a much better performance with the 5850 or 5870

go with a gpu upgrade. also a 5870 is cheaper than $500 CAD

Get the GPU upgrade for sure. If you can afford the 5870, get it.. it's a beast. Perfect card for 1080p IMO. Your system should perform very nearly identically to mine once you get it.