All else being equal, your choice of motherboard will not significantly decrease your rendering time. Neither will the amount of RAM, assuming you aren’t hitting your page file (In WinXP 512mb is enough; 1g is better).
Yes, if your only intended output is DVD then you may as well render straight to mpeg2.
Vegas is already optimized afaik.
Don’t expect to see a 4X decrease in rendering time unless you too are coming from a much older CPU.