Ok, here's a real answer. All PIII Coppermines are on the same core, so, except for random differences between chips, they all have more or less the same headroom. Most PIII's only overclock to around 933 or less easily. Some go higher, but the probablity of success gets lower and lower. Luckily, a 700 overclocks to a nice 933 when the FSB is 133. It's convient because you can do it without overclocking PCI or AGP or memory (assuming you have a 133 capable board and PC133 memory). The 800 wants to go to 1066 on a 133 bus - you'll never make it. So the two chips will go to around the same frequency, but the 700 does it with a convenient FSB which also runs the memory faster, so it will be faster at a given clock speed than the 800.
Hope that wasn't too much to handle. Get the 700, trust us on this one. It will be faster.