Wow...is that a typo or are you just dumb?
The 7800 architecture can't even reach 625mhz core, much less 500mhz. Also, why would you underclock a X1800 XT?! 430/1200?! Why would you do that? It would be like buying a McLaren F1 GTR and taking out the V12 engine and replacing it with a 4 cylinder Ford Pinto engine. Bascially, it's stuuupid.
I'll give you the benefit of doubt though...I think you just had a typo there, so it's all good.