Hey
freshspace, here's a bit of a rundown on How Things Are:
The motherboard has a sort of "reference signal" called the Front-Side Bus (FSB). In your case, the stock FSB is 66 megahertz (MHz) because that's a characteristic of the Celeron CPUs that are made to run at 766MHz or less.
The PCI bus runs at a ratio of the FSB. It hosts the hard-drive controller, along with any expansion cards you plug into PCI slots (such as sound cards, network cards, modems, etc). The PCI bus on your motherboard has a "home" frequency of 33MHz, and so it runs at a 1:2 ratio of your 66MHz FSB. If you had a CPU that uses a 100MHz FSB, the PCI bus should self-adjust to a 1:3 ratio so it stays at its specified 33MHz.
The AGP port also runs at a ratio of the FSB. Its home frequency is 66MHz, so with your CPU it would be a 1:1 ratio. If your CPU used a 100MHz FSB, the AGP port would run at a 2:3 ratio.
The CPU itself
also runs at a ratio of the FSB. The CPU's ratio is called the "multiplier." For a 700MHz Celeron, the multiplier is 10.5 and the CPU runs at 7.5 x 66MHz FSB = ~700MHz.
The memory (RAM) also is referenced to the FSB speed. Memory is built to run at a specific speed and can almost always tolerate being run slower than it was designed to. PC133 memory, for example, can run at 133MHz, 100MHz or 66MHz. There are different implementations of this but basically, the memory is going to run at some ratio of the FSB speed, and often this ratio can be independently dictated in the BIOS. If you had a 66MHz FSB, you might have the option to run the memory at either a 1:1 ratio (66MHz) or a 3:2 ratio (100MHz). If you start raising the FSB speed, the memory speed will maintain the ratio and speed up proportionately.
So the key is the FSB, and the ratios. The Celeron is multiplier-locked, or else you'd be able to raise the FSB while lowering the multiplier and have way too much fun. 
You don't want your PCI bus to run too far from its home frequency of 33MHz or your hard drive may have Problems. Let's look at some examples using your 700MHz CPU as the basis:
Example 1: NuB user tries to overclock to 83MHz FSB
- FSB = 83MHz
- PCI = 1/2 of FSB = 41.5MHz Red Alert, she canna take it for long, Captain! :Q The hard drives are corruptin' from the high PCI-bus speed! :Q
- AGP = 1/1 of FSB = 83MHz A good AGP card can hack it here, but it's not easy on it.
- Memory = 1/1 of FSB = 83MHz If the memory is designed to run at only 66MHz then this is an issue. If it's PC100 or PC133 then it's still breathing easily
- CPU = 10.5 x FSB = 870MHz The CPU is happy because this isn't too much of an overclock, but the rest of the system is in distress, except possibly the memory if it's built to go faster.
Example 2: NuB user studies issue, and uses ratios to his advantage
- FSB = 100MHz
- PCI = 1/3 of FSB = 33MHz Perfectly on-spec because the mobo automatically kicked the ratio down to 1/3 when it was set to 100MHz FSB
- AGP = 2/3 of FSB = 66MHz Again, right on target
- Memory = 1/1 of FSB = 100MHz If the memory is PC66 then this is going to be pushing it, but if it's PC100 or PC133 then it'll be happy.
- CPU = 10.5 x FSB = 1050MHz The CPU is probably going to need an extra 10% more voltage to do this, and reasonably good cooling, but the rest of the system is on-spec and happy.
So you can see that if you stray too far from one of the motherboard's "home" FSB frequencies (namely 66MHz or 100MHz) then things could backfire on you even if the CPU itself is happy at the speed that resulted.
To really answer the question about why flashing to a new BIOS might help overclock... Some boards allow for voltage adjustments and manual manipulation of the PCI and AGP ratios, giving you more room to play with, and
sometimes a BIOS update adds these features, but in the case of your motherboard, I don't think it would because it's just not aimed at a hardcore tweaker customer.
You DO have the ability to change the FSB in your BIOS, however, and so your safe target would be the 100MHz FSB. If your CPU will handle that, all the other stuff is going to be on-spec. The question is whether your CPU can do it at stock voltage, since you don't have any voltage adjustments available.
When you do look in the BIOS to adjust the FSB speed, oftentimes the speeds will be shown as FSB/PCI... for instance,
66/33
75/37
83/41
100/33
105/34
113/38
133/33
and this is intended to help figure out what the effect will be on the PCI speed, which is probably the "gotcha" to watch. The AGP bus is not so critical.
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