I suggest a cheap socket 754 motherboard to pair with your video card. You can put a cheap s754 cpu like a 3000+ and that will give you much more power than a 939 with a crippled mutant AGP port running your 6800 like a 5700....
After you get the most of that system, then sell the whole thing and get the best and latest that money can buy at this time.. Maybe new socketM@ from AMD with improved DDR2 bandwidth and dual core...
The goal is to spend the less now to get the best later. And no.. socket 939 is not a futureproof option.. it will leave the scene just like the others in 2006..