Dont you have a computer shop somewhere, you could go there and ask them to build you a custom computer... you can usualy just take all the things uve bought there and they will put it together for a reasonable price.
Yeah get a DFI Lanparty motherboard, they are very good. You should also think about getting G.Skill ram, very good for overclocking, so is the DFI motherboard, the AMD 64 3700+ is a good processor to overclock aswell.
Oh and finally custom pc's r always better then all the other big brand computer companys, if ur gonna be doing hardcore-gaming like you said, you dont wanna go buying one of those systems coz they are a rip-off.