For your stated budget of $400-450, the biggest performance gains you will see will be from an upgraded videocard & processor. Because your current board is not much different than an Asus A8V (same chipset), I would keep it. Because your board is AGP compatible, a really strong AGP card matched with a new processor would be a sound upgrade path for gaming and daily stuff.
Some strong AGP vid cards would be, separated by price:
Geforce 7800GS 256MB AGP $290
Geforce 6800XT 256MB AGP $143.
Radeon X800XT 256MB AGP $270
Radeon X800GTO 256MB AGP $141
Along with a good processor:
AMD Athlon 64 3200+ $122
AMD Athlon 64 3400+ $150.