Overclock gtx 295 safe limits

Hi

I was wondering what would be safe limits on overclocking my BFG GTX 296. at the moment it is running

Clock 576
Memory 999
Shader 1242
(defaults)

The rest of my system stands at
CPU: i7 920 @ 3.26ghz
6gb tripple channel corsair @ 1284mhz - overclocked with gigabyte software
MOBO: gigabyte - X58 ud4
Case: Antec 900 - 2

I was tinkering with rivatuner but i am new to overclocking and wanted to know what are safe limits in terms of temperature and is there anything else i should be aware of

The temps i have at the min (idle) are:
GPU - 58c
PCB Temp: 55c


I hope you cn help

Thanks

KV3