If anyone knows FX5200's, it's me.
You have to tick the box or whatever that says, apply these settings at startup. It's hard to notice the difference but you see it in game that need power, you'll get about 2 extra fps per every 20fps or some.
On the XFX FX5200 in my Athlon2000+ rig i got clocks of 294Mhz core and 464Mhz Mem from defualts of 250Mhz and 333Mhz
On the Albatron FX5200 in my mums P4 2.8 Northy rig i got clocks of 290Mhz core and 450Mhz from the same defualts as above. This one scores higher though becuase of the 2.8 Northystylin.
I think you've pushed your core far enough, keep trying with your mem, expect at least a 70Mhz increase, it could reach 500Mhz mem.
Overclocking yuor card is perfectly safe, not 1 reported card has been screwed through overclocking, it is basically impossibe. Heat and Voltage are the 2 killers but they will only kill if they are put to ridiculous levels. If you FX5200 busts, i'll give you mine, thats how much i guarantee it will not die.
Even if you put it up to 800Mhz on both core and mem. It will just freeze and you'll have to reboot. When something freezers, instead of being a processor (GPU) it turn into like a power line on your card and power simply passes through with minamal heat due to it's free flowing. The memroy simply rejects everytihng once the GPU has frozen.
Hope i've helped man!!!