Just get a 670 2GB, it'll be perfect for what you're talking about doing with it. I recommend either the Gigabyte Windforce 3 or the EVGA FTW model. There's other good ones, but just to keep it simple, those are two great options that a lot of people here own, and have been happy with.
As is mentioned above, the 'stock' clocks are based on an absolute lowest common denominator amongst ALL the chip samples, along with various technical and business concerns. As such, any individual card sample is extremely likely to have *some* room to safely OC.
It's possible to harm a GPU if you overvolt it too much, but a simple overclock at stock voltage cannot hurt your card. The worst possible scenario is that you corrupt open files (such as OS files) by having your computer crash at an inopportune time due to the OC. But this is pretty rare.
While there's no compelling reason NOT to overclock, there's really not a particularly compelling reason TO overclock a 670, either. They don't generally have enough headroom (i.e. the achievable increase in clocks above the stock clock, measured as a %) for you to be able to achieve an OC large enough that you could 'see it' with the naked eye while gaming anyways. OC'ing on the 670 is mostly going to be valuable for 'e-peen' i.e. benching, not for practicality in terms of being able to increase your 'max playable settings'.
There have been other GPU's in history that have enough headroom that you could OC enough to make a visible difference (GTX470 and HD7950 are recent examples), but the 670 is generally relatively close to it's max clocks right out of the box.
As such, you should get one and run it stock for now.
Lastly, spending $40 to have someone install your GPU is a horrible waste of money. It's seriously like a 10 minute job ... 5 minutes to do the hardware part, 5 minutes to do the software part. It's painfully simple to achieve, esp. if you already have a GPU installed so you can see how it's already been done ... there's 11 years old on these boards who do it all the time. You can handle doing it yourself, trust me