I agree with Grey. I'm in a similar scenario- I really, really, really, really, really, really, really, really, really, really, really, really
want to upgrade my video card. However I can't really say it is out of need. For all intents and purposes I can play every game out there if not on the highest in-game settings then pretty darn near them. So as much as I may *want* to upgrade, I'm thinking I'm gonna' hold off until I can purchase a single card that will provided a 100% performance increase across the board consistently for around $200 or so. Probably gonna' be waiting for another year or two.
Also might pickup Bob's 5870 for some CrossFire goodness as that'd most certainly tide me over while I wait the next two years.
You CPU, honestly, isn't going to be that much slower than mine if I have turbo disabled and HT off (which I do) so it's should be plenty to push most any game, especially if you OC that CPU (mine isn't).
I'd take the time with FRAPs and experiment with all your favorite games. Play them with everything bone stock. Then play them with that 580 oc'd. Then play them again with the 580 stock and the CPU oc'd. See which component when oc'd offers the greatest increase across the board and that'll let you know which one is holding you back. And then for kicks, try those games with both your CPU and 580 oc'd. Who knows, maybe you'll find that with both of them oc'd you really wont need to upgrade and you can either save that cash or put it towards bonus points with your lady.