I don't overclock or tweak my card in anyway so I don't use CCC at all, I only install the driver. Been using ATi for a decade since my Rage Pro Turbo from 1999 (that card still works in a friend's PC!), no driver issues whatsoever.
My Asus 4850 512 MB has a fairly large Glaciator heatsink...
He did say earlier.
So yeah a 5770 seems like a good, sensible upgrade. Depends on the power supply though, I'd say at least 25A total on the 12 volt rail.
Two days ago I took out my 4850 and gave the heatsink a vacuum as it was very dusty. Then after reinstalling the card my PC wouldn't POST as I couldn't get any display. The card was working just prior to removing it. I thought about it for a while, then took out the card again and gave the PCIe...
If you're running Vista, it's not ATi's fault, because Vista doesn't support two different display drivers running simultaneously. You can do that in XP or 7, but just not in Vista. Those of you who are playing around with an nVidia card for Physx and ATi for rendering should know this.
Edit...
I wouldn't use any molex connectors if possible but if you have to, plug both in to be safe.
A good power supply for the GTX275 / 4890 would be this 550 watt Corsair, http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004 but if you don't care for rebates the 650 watt model is for...
EVGA, XFX and BFG for nVidia. XFX for ATi.
It's good that you're willing to try ATi, I was expecting the "I don't touch ATi cards" response :D
Personally, I would take this 4890 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814150359), unless you could find any cheaper GTX275s...