I have a core i5 with 4gigs of ram at ddr3 1600 and I left the bios alone and it runs at 3.2 when the load requires it.
If the cooling is right it should be fine.
Personally no. 2 reasons one becasue its from ebay...always a bit dodgy
and sencond, if you want an overclocked system do it yourself it isn't really that hard and then at least you will know what is going on. I wldn't really trust an already overclocked system, tehre are too many things taht could go wrong, and you won't get warranty at aleast on your CPU, becasue it was overclocked and on the proof of purchase probably says so. Aprt from anything else they would have to ship the system all built togetther and the way teh courires send stuff....dunno, it just dosent sound like a good idea...and anyways six hours ain't really enough of a stress test for an Oc system, most people say at least 12 to be completly sure it is stable.