You're not really cutting it close. A system with a highly OCed, power-hungry i7 965 drew 279W at full load with a GXT 670 and you're system will draw less power. Review: http://www.guru3d.com/article/geforce-gtx-670-review/8
Indications of insufficient power:
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
You don't need to upgrade your PSU and you actually have some room for OC. If you wanted to buy a new PSU to use money that was burning a hole in your pocket, then I wouldn't have recommended anything more than an XFX Core 550W with 44A on the 12V rails. If you paid 48-49 pounds for that OCZ, then you would have gotten a better deal if you had gotten the XFX in the first place...50 at Amazon and Scan and free shipping at Amazon.