According to the listed specs both list DDR333 as their highest supported DDR speeds.
The P4G8X does offer locked PCI/AGP, but it does NOT offer the ability to clock the RAM seperately from the FSB. This means that your RAM speed is locked to your bus speed. This is fine if you have cheap RAM, but if you just got a Corsair 3500C2 in and you want to see what it can do, this board wont let you do much with it.
The P4S8X allows you to change the memory settings. You can run the 1:1 multiplier, 5:4 (for DDR333), or 3:2 (for DDR400). Notice however that the 3:2 has very limited functionality. If you run your bus as stock 133 with decent RAM you will be fine. But as soon as that bus goes to 150 you need RAM that can handle a 225 (450DDR) bus and there arent many out there that can do it. So odds are you are going to have to use the 5:4 multiplier anyway. In my opinion a 4:3 multiplier like my P4PE has would be a lot more practical than the 3:2. Another note, PCI/AGP are NOT lockable and are based on your FSB speed.
Personally, if your choice was only between these 2, I would take the Intel. There was another thread around here about SiS and there are a couple decent posts there.
I would pose the question of why your selection is limited to these 2 though. The SiS doesn't really have anything that you can't get on another board and it doesn't even support dual channel DDR. The Intel does support dual channel DDR, but the numbers are showing that dual channel really doesn't do much for you and the P4G8X isn't that great a board for overclocking.
Not trying to label my board as the greatest, but the only thing the P4PE is missing is 8x AGP and most reviews you read will tell you that 8x AGP really doesn't mean much right now.
Each board has it's place, but I think as far as Asus offerings the P4PE is the only P4 board that is a really good choice for enthusiasts right now.
