If your Radeon does all you want why upgrade?
Besides, the R8500 is in most games reasonably close to the Ti4200 already and you've got yours overclocked a decent extent too.
Seems to me you don't stand to gain terribly much in terms of performance, and lose out in a few other respects.
Plus with ATi announcing a new product in a few weeks it seems better to wait anyhow.
Unless whatever ATi has coming is damn impressive I'd vote you stick with your R8500, it should be more then capable for now.