BFG 7600 GT Overclocking. "I Need Help!"

Hey guys, I will give you my specs before I ask the question.

Core 2 Duo E6600
2 BFG GeForce 7600 GT OC (SLI)
2GB Corssair XMS2 DDR2 800
700w OCZ power
eVGA 680i Mobo
320GB Hard drive 16MB cache 7200RPM

So what am I doing wrong. I am trying to overclock my cards which are default core 580 and memory 725. In my windows NVIDIA control panel program I have overclocked them to every setting I can think of including the max overclock setting of 1160 core and 870 mem. That seems to be the max on my slider to overclock it. I have now changed it back to default at the moment. I have been testing it with max graphics on Half Life 2: Episode Two and picked a really graphics intense area and I haven't seen any difference at all. Am I missing something here? Do I have to restart my computer each time? And yes I did change the settings and hit apply each time. I am new to overclocking, so please let me know. Also, please let me know your recommendation on a good overclocked setting for my computer specs. I have plenty of cooling and good temps at idle of:

CPU: 37
SYSTEM CASE:30
GPU 1: 42
GPU2: 46

Without overclocking the temps dont change by much under load. Great cooling system.