I don't think that a new mobo alone is going to do much for your systems performance. Well, unless you have some OCing in mind or you BX6 is defective. Don't know what your system is but maybe you could gain more by spending your money to upgrade other components. New vid card? More ram? Faster hdd?
Since BX performance is fairly good compared to via and i815, and you must stick with your slot 1 cpu, you might want to think about sticking with your BX6 until you're ready to upgrade your cpu, then buy a mobo that best fits your new cpu.
Good BX systems still smoke man.