Although one thing to be aware of is that you will need faster memory for overclocking the e8400 as it starts at a far higher fbs speed.

ie e5200 (fsb 200mhz)+ pc6400 memory (fsb 400mhz)= the cpu will hit it's overclock cap before the memory does.
e8400 (fsb 333mhz)+ pc6400 memory (fsb 400mhz)= the memory will hit it's limit long before the cpu hit's it's limit.
generally speaking all the core 2 chips seem to be able to get a 1gb overclock
so in theory you'll be looking at roughly around 3.5ghz for the e5200 and 4ghz for the e8400 (assumeing the motherboard and memory can cope)