how much better does a 5800 look compared to the 4800 right now?

Image quality isn't going to change much, but performance may depending on what games you play and at what settings.

I previously bought an XFX HD 4890 XXX and it was a great card but I exchanged it for a Sapphire HD 5850 and the biggest differences were:

Graphics Quality and FPS: Crysis was like two different worlds when played with the highest setting on my 24" monitor. Even a bigger difference when played on the 27" monitor at my office. The 5850 really killed the 4890, which would show a really subtle lag every once in a while on Enthusiast settings with AA on.

Fan Noice and Temps: The 4890 XXX fan was really loud, and the temps never got as low as they get on the HD 5850 under load. The 5850 gets really low temps with just a little boost over the factory fan settings. Still quiet too.

I changed hd4870 with 5870.I noticed huge difference in terms of fps.Also now i have low tempretures and dx11 support.

5870=4870x2 so it may not be a drastic change for you it would take you down to a single card setup with more upgradebility. All while saving power heat and noise.