With every chip being different we cannot give you an accurate measurement to how high you can overlcock. Depending on your motherboard and cooling, that'll make most of the difference. But you could also just have a bad OCing chip. It's the luck of the draw.
While Nassor is correct in talking about voltages, he is wrong in saying that with a stock cooler, you won't want to raise the voltages too much. And let me explain why. Firstly, the process is cooled obviously by the heatsink coupled with a fan. Stock voltages usually range different for every processor, especially on low TDP chips. The VRMs (Voltage Regulator Modules) reside outside of the CPU, usually on the motherboard. The only way for these to get cooled off, is for air to blow across them, or a heatsink be placed on them. But as he said, watch your voltages. Your MOSFETs (Metal–Oxide Semiconductor Field-Effect Transistors) also tend to take a hit when you up the voltages, though higher end motherboards place a heatsink on them as well. Though getting an aftermarket cooler will help immensely for CPU temps, it will not decrease the temperature of the VRM's on the motherboard you have or the MOSFETs. Raising voltages will however raise your CPU temperature slightly, so stay on the safe side.
When overclocking, you're going to want to watch voltages as well as your multiplier and FSB, along with your northbridge voltage if you have that option. Your memory will usually limit you if it runs at a speed of 1066MHz or even 800MHz, though when it comes down to it, the timings you have the RAM set at are the biggest factor in limiting an overclock. You should be fine though with RAM running at 667MHz.
Concerning your current temperature, that's not too bad, if at around a fifty percent load. If by chance that is idle temp, that's probably a bit too high to start overclocking anything. Your load temperature (processor working at 100 percent) is probably nearing the mid 65C area I'm guessing, which isn't all too good. I would make sure my temperatures don't hit any higher than 70C with an E5300 if I were you, since your processor has a thermal specification of about 74C. While Intel recommends that you don't take the voltages any higher than 1.3625V, I would personally take her to 1.4V and call it safe with proper case air flow. Just make sure the cooling for your case is adequate.
Stability also counts. You'll want to monitor your temps with a program such as Real Temp or Core Temp, though I usually only use Core Temp to monitor AMD processors, and Real Temp for Intel based platforms. Once you have an overclock you feel comfortable with, run some stability programs such as Prime95 or OCCT. Even IntelBurnTest will allow you to test stability. I recommend running Prime95 for about 8 hours at first, and if you want to raise your overclock after it passes, then do so. If Prime95 fails the test and one of your cores stops working, that usually means either your RAM timings are set to tight, or your RAM is running too fast. Though it could also mean that either your northbridge voltage or CPU VID isn't high enough. But as always, watch temperatures, don't go crazy on voltages at all, and when you think you've got a final overclock, I'd let Prime95 run anywhere from 16 to 24 hours. I usually do a 24 hour test to make sure the stability is there for a 24/7 use.
Also, I'm not sure if you built the computer yourself, but if you have a motherboard that was in the computer when you bought it from a company such as HP, Sony, Dell, etc. you won't be able to overclock. Manufacturers lock those settings from consumers. But if you have a motherboard from building the system yourself, you should be able to tweak it a bit.
FINAL NOTE: DO NOT OVERCLOCK YOUR COMPUTER WITHOUT KNOWING EXACTLY WHAT YOU ARE DOING. WARRANTIES ARE ALMOST GARUNTEED TO BE VOIDED WHEN TAKING THE SPEEDS HIGHER THAN STOCK.
