thinking of upgrading mobo.......need some advice.

hello everyone.
I'm am thinking of upgrading my mobo and need some advice. I am somewhat new to building computers so bear with me please. Our computer that we have now is one that i built. but its starting to get out dated. I think i will be looking for a mobo/cpu cobo. What i have access to right now to put on the mobo is:

An extrememusic audio card
An geforce 6600gt 256mb/128bit ddr2 video card
2gigs of 184 pin ps2100 ddr ram
650 watt power supply
dvdrom
cd r/rw drive
and two hdd

Price is somewhat an issue. Im thinking somewhere in the neiborhood of $200. Im hoping to be able to get a good stable mobo that i will be able to upgrade in the future. Any direction would be of great help.

thankyou for your quick reply.
Im not real sure on the cpu. my wife loves to listen to music, and the kids love to play games. But right now the games that they are into are games like civ 4, warcraft3, empire earth2, and the younger one like to play runescape online. I am usually all over the place , alittle 3d editting, video editing, mixing audio, and of course playing games, watching movies and such. Ive seen alot of mentioning about dual core coming out but do i really need something like that at this moment??????