Above is ASUS CPU support for your mainboard. C2D's are not listed on this page. Board has only 800 FSB support so only 2000/4000 Allendales may be outside chances to run if voltage is supported. Conroe requires at least 1066 FSB. Intel 915G appears to have some limited core 2 support, as your board sports a 915 chipset you might just get away with one of the above CPU's.
However in my experience mobo platforms (chipsets) are more critical to C2D experience where as AMD's with on board memory controller perform pretty well equally on all platforms. So I'd recommend upgrading the board as your current one won't get the best out of the CPU even if it ran on it at all.