It's not as massive as some people are purporting it to be, but it is large...
It's like going from a GF4 Ti 4200 to an 8800GTX. You get the next generation of DirectX support, and about 3x the performance.
I've had a GF2MX, GF4Ti4200, 7900GS, 8800GTX, HD5870, GTX 580,GTX560 Ti x2, 6970x4 and I must say the 580 vs the 8800gtx enabled me to play a few games I had been wanting to play which is exactly what you want from a graphics card. I usually hold off on upgrading until there is a game out that I really dig and my current setup can't keep playable framerates maxed out with no AA at 1080p.
The 580 will allow you to play a few games that you couldn't otherwise play with the GTX, but if you're willing to crank a few settings down, you could get by with it until the next wave of cards hits and the prices come down on the 580 even more.
A ~$250 580 would be THE bang for the buck for sure.