If you're going P4 I would suggest not going with an SiS based motherboard. IMHO, I've never seen anything good come from them. Performance with SiS chipsets seems to have always been under par and I've seen several compatibility problems. SiS boards have always been cheaper. I tend to believe you get what you pay for. Of course, I wouldn't pay $500 for a graphics card. That's just nuts.
Im gonna stick up for SiS here bc i have an old P4 system (2.0A) using an Asus P4S333-m board which is based an a SiS chipset and it is rock solid, has been for about umm...its almost 3 years old now (wow is it really that old???)? Now only that, but it was one of the first, if not the first, board to support PC2700. Never had any issues with it.
That being said, i dont know how there current chipsets fair compared to Nvidia's, Via's and Intel's.