Its all in what components you have.....You state these figures yet you do not state whether they are stable settings? If the RAM is stable at the higher 258 FSB without underclocking it, then I would opt for the OC on the 258x13 and give up 50 MHz of speed on the CPU...When using the FSB it is essentially the volume button for everything in your system i.e. : CPU,RAM,HT,NB..When going up with FSB the NB and RAM will become unstable before the CPU..thereby lowering the FSB and raising the CPU multiplier will allow for the CPU to get stable at higher clocks due to the fact that your not turning the RAM,NB, and HT up making them become unstable at the higher BCLK...
Both are very stable settings. But if i do 258x13, i have to increase the RAM voltage/timings and its frequency, and the cpu temperature 33'C - 38'C. If i do 227x15, I dont have to configure the RAM timings/voltage and its frequency, but the cpu temperature can reach 45'C.
You didn't say, but I am guessing that you have an E5200 series chip.
Generally, what Moto suggests is the easier way because you reduce the effects of instability on the memory system.
That is the approach I take.
GA-EP45-UD3P | Q9550 OC'd to 3.6 GHz (425 MHz X 8.5) C3 stepping
GA-EP45-UD3L | Q6600 OC'd to 3.6 GHz (400 MHz X 9)
GA-EP35-DS3P | E7500 OC'd to 4.1 GHz (373 MHz X 11)
GA-G41M-ES2L | E6500 OC'd to 3.87 GHz (352 MHz X 11) FSB limited by G41
(I like Core2 CPU's and Gigabyte motherboards.)
Theoretically, the higher FSB freq X lower mult is better. In reality, for a given core speed, particularly with the Core2 CPU's, performance does not scale linearly with FSB freq. That is a fancy way of saying that performance at the higher FSB speed will not be 12% better.
Assuming that you do have an E5200 series chip, things are a little different because of the lower 200 MHz FSB freq. There's less stress on the memory.
Your core temps seem to indicate that you are using the stock cooler. Even a mediocre aftermarket cooler will help a lot because a dual core chip is much easier to cool than a quad.
You need to keep the load temps when running something like Prime95 under 70 C.
The E5200 chips are pretty good overclockers. With good cooling, many will reach 4 GHz. Mine unfortunately only goes to 3.87 GHz.
BTW, it's a coincidence that my E6500 does that in a G41 board.
i have ga-g41m-es2l rev 1.1 motherboard with dual core e5700 3.00ghz. I did run prime95 with no problem, 52'C - 55'C (258x13), and 62'C - 66'C(227x15), checked with CPUID HWMonitor during the full stress of prime95. 258x13 s the best i can do (including maintaining the cpu temperature). I think it is because of my ram, right. Corsair 2gb ddr2 800mhz combined with kingston 512mb ddr2 667. I have tried increase the ram frequency but no success. Or am i forget smething?
You only want the ram to be running at 400MHz in your bios screen,
DOUBLE data rate makes it 800MHz, I suspect your overclocking your ram there
try setting your ram speed to the closest to 400MHz (it'll probably be 333MHz or thereaboutsif its presets) the bios will let you,
then see about squeezing a little more out of the chip
and +1 on the better cooler ofc, overclocking=more heat
i hve bought better cooler, cooler master dual fan (for now just 1 fan installed) to replace the default cooler. It did a good job. In the bios, there are 3 multiplier for the ram, 2.66 , 3.33 and 4.00. I was stuck at 2.66.
Cpu clock ratio : [13x] <---15 highest
Fine cpu clock ratio : [0.0] <---0.5 highest
Cpu frequency : 3.35ghz (258x13)
***CLOCK CHIP CONTROL***
Cpu host control : [enabled]
Cpu host frequency(mhz) : 
Pci express frequency(mhz) : [auto] <--- auto is 100mhz
Yup, your overclocking your ram, overvolting it and timings are possibly too much too stick the auto button back on for them and see what the system decides to run it at,
Aah, I also see you have mismatched ram, get that 512 out of there
your 800MHz ram is being slowed down by that for a start,