Quote:

would my old DIMM sdram (333Mhz DDR ...not DDR2) work on a ASUS mainboard designed for DDR2 667/533 MHz ram (of course only with 333Mhz!)??


No, DDR2 is not backwards compatible to DDR.
I have had some bad experiences with an ASUS board recently and many others were complaining that their memory did not work (in my case the board was broken and the replacement is now working flawlessly). If you can go with Gigabyte I would recommend those instead of Asus. Either way: once you have picked a mainboard check their website to find out the EXACT brand and type of memory that they support and order that. Don't buy the memory first and then some mainboard.

As for multi-core support:
games are not too well suited for using multiple cores but some do. The important part is that intel has switched its consumer lines almost exclusively to multi-core so you can expect more applications to make use of that. Even without multi-core application support at least the OS can be offloaded to a secondary core but I don't know how much of a speed gain that will net.