The ATI 5850 is between the GTX 460 and GTX 470 performance wise. If you already have an ATI 5850, I see no point in upgrading unless you wanted to add a second one for Crossfire setup.
There's really no reason an ATI 5850 shouldn't work well for any game that's current available. What resolution is your monitor?
Edit: BTW, the i7-930 + X58 motherboard upgrade certainly can't hurt. The i7's perform much better than the older Core 2 Quad CPUs if that's what you have. However, keep in mind you'll also need to upgrade your RAM as well with the new motherboard. I'd still stick with the ATI 5850 you already have though.