That card looks scary - all those skulls.

I just went to a 4850 about a month ago for 4 and 5 boxing and am very happy. I also put a 9800 into my daughter's computer yesterday and it is very nice (no boxing but could handle it). You will probably be happy with the card you are looking at (skulls or not).

That said, do you see performance issues now? Get a trial account going and test with 5 and see what it looks like. You current card is very good too, a lot of people here seem to 5 box with it just fine. You might actually run into more issues with a dual core CPU (vs Quad core) or 4 GB of RAM (vs 8GB 4 should still be ok generally provided 64 bit OS) before you hit issues with your GPU.

Test with 5, for free, see how it goes. Go to cities and see what happens. Report back here with results and get further recommendations based on what happens. Then spend your monies.