Usually higher fsb means more performance boost than raising cpu speed with multiplier.
So if your board can run 300 on the fsb (it probably can't) and you run 10 multiplier,a lot of the non-locked ratios will gain the 50% boost in speed. If you run 250fsb and 12 multiplier the other stuff (chipset) will only get a 25% boost while the processor runs the same.
Now I don't remember the max multiplier for the 5200, so exactly what to do I can't tell you. But try lowering HT and memory speeds and set multiplier to 8 or something. Then figure out how high your fsb can get. After that, figure out what multiplier you want to run. ie. if 268fsb is max, you'd want to run HT at 600 (stock 800) and cpu at 11 multiplier to get to the 3ghz - if your components are stable that is. But I'd suppose you're smart enough to raise multiplier 1 notch at a time once you've foudn the limit for fsb.
ps. run dual prime95 for at least 10 minutes every time you think you've found a stable speed. It can still be unstable even if it passes 10 minutes, so try 12 hours or so once you're done. (1 small, 1 blend)
pps. remember to :
disable cool'n'quet and fan controls if any (run 100%)
lock the pci and pcie speeds to default 33 & 100
underclock memory and cpu till fsb limit is found, then back 2mhz or so down before finding cpu limit. Disregard memory till last.
Avoid upping the vcore if possible - keeps the cpu cooler.