You're not going to get a game changing experience with the extra clocks on offer and when you look through the graphs you see that the performance increase is only a couple of FPS here and there, which doesn't sound all that impressive.
When you start to break down the numbers and figure out the performance increase in a % form, you realize that the numbers aren't too bad at all with increases being around 10%. You're hardly going to complain about an extra 10% performance at the cost of nothing.
While I heard someone say that the overclock they achieved through Overdrive bought performance up to the higher end HD 5750, you can clearly see today that it just isn't true. The drop in stream processors impact overall performance quite heavily and you're just not going to be able to achieve enough MHz in the core and memory department to combat that and bring performance up.
While you might think that an extra 10% performance isn't a whole lot, when it comes to these mid-range cards it can be the difference between moving from medium to high or turning up another detail and making the game continue to run smooth.
The HD 5670 already represents good value, although you can see today that the value is slightly better again. If the HD 5670 is in your budget, it's worth knowing that you're not going to be limited to old games, but more so just some lower detail.