1GHz on this card is a bit of a stretch. Seeing as how you are already running at 880MHz (which is very high, TBH) you aren't going to go much further.
In the newest version of MSI Afterburner, when you unlock the core voltage, it allows you to press it to a certain maximum. The GTX460's becomes unstable at around 950MHz with the core voltage maxed out in Afterburner, so I'd suggest you don't try to reach 1GHz. Shader clock is double the core clock (by default), and I'd suggest you don't mess with it. Also the memory clock isn't worth upping, so don't even try it (they don't have decent enough cooling anyhow).
I have the MSI GTX460 Cyclone, and I've reached 925MHz with the stock voltage (unstable as hell). I haven't tried upping the voltage yet, but I don't intend to. The performance increase I noticed between the 800MHz I'm running at now and the 900MHz I got with stock voltages (ran more or less stable) isn't worth the worries about temperature, stability and things as such.
I'd suggest you just leave the card as is, 880MHz is very high already. Memory clock is high too (mine is set to 1900MHz), so you'll get basically best performance.
Before you try to OC, make sure your load temps are ok. Run FurMark with GPU Shark open in the background for 10 minutes and see what maximum temps your GFX card reaches as it is now. FurMark is free AFAIK, small and very easy to use. Run it at it's maximum resolution with AA set to 8x and see what happens. You don't want to OC with an already hot card, although I think the TwinFrozr works macigally on the GTX460 GPU. Best be safe though. Post your current idle temps and maximum temps after the test here plox, so we can have a looksie!