Asus P4S533-X, RAM question

Asus P4S533-X mobo,Intel P 4HT, 3066 Mhz, sis 645DX, 1024 MB (DDR SDRAM ), NVIDA GeForce FX 5200 (128 MB), WDC WD800 JB HD, Lite-on Combo LTC-4816 CD/DVD running WinXP Pro, I have 2,DDR SDRAM slots filled with Corsair 512 XMS DDR, and have 2 SDR DIMM slots not filled, can I add
2 512 SDR DIMMs into these slots while still using my DDR DIMM slots filled with DDR.
The DDRs are Corsair 2700, 166/333, but only running at 133, how is this controled and can I manualy change BIOS, AWARD Modular(02/04/03) or is this something set automaticaly by Mobo or BIOS?
Thanks for any input, this is old computer , first one I ever built about 2 years ago, I'm over 60 years old and things don't stay put in my mind like they used too.
Hope this is all info. needed to answer questions. I almost forgot US Robotics 56k V.92 faxmodem, I live in the country and thats all I can get, land line!

 

Thanks for info. Halo