Yes, 1.58V is a little high for that level of overclock. I'd drop that down to 1.45V and see if it will run at that voltage level. Which should drop your temps down a bit too.
As for instability past 275MHz, that could be your northbridge overheating. Do you still have the stock heatsink on that? I'd recommend upgrading to a better cooler on there in addition to a new CPU heatsink, because that could be one thing that is limiting your overclock. Stock heatsinks aren't typically known for cooling well and/or running cool.
On the memory you could try raising the voltage to 2.75V. Really the kind of performance and speeds it will run at depends on what kind of memory chips it has and what they are rated to on their maximum frequency (speed). Really I'd be surprised if they don't run at the 333MHz multiplier at some settings of some sort. Also, you could try running looser timings at say 3-4-4-8 or 3-4-4-10 rather than 3-3-3-8 and see if that works.
I'd say try the x4 multiplier for 1100MHz and see if it is stable at that frequency setting.
