I have two E5700 and their overclocking speed are different. The old one that I bought two months ago oc better at 15 x 265 = 3975 MHz with 1.376v vcore (cpu-z) and max temp 65C (core temp, prime95) installed on a low end motherboard Biostar TP43D2-A7. The new one that I bought yesterday can only reach 15 x 250 = 3750 MHz with 1.408v vcore (cpu-z) and max temp 67C on my Gigabyte EP45-UD3L mobo. Both are using the same cooler, Cooler Master Hyper 212 Plus. My better E5700 can reach 4.1-4.2 GHz, but I don't like temperature over 70C. I've just found out two of my cpus (conroe E6700 and E5300) have degraded their performance after two years running in constant full load (doing BOINC) with over 70c temp almost 24/7. I was using stock cooler. The E6700 can even run stable at stock speed anymore. That was the reason I bought new cpus.
So yes, the limit of E5700 seem around 4 GHz ± 10% within reasonable vcore and temperature. Use quality cooler and don't go too much above 1.4v if you plan using it longer than 3 years.