A few notes on upgrades:
With very, very few exceptions, motherboards you purchase now require DDR2, which is incompatible with DDR in voltage, signaling and module pin-count (184-pin DDR, 240-pin DDR2).
While not absolutely required, depending on the power supply you have now, it may be a worthwhile investment to purchase a new unit, with more modern specs. Something from FSP or Ultra should afford you a nice 500-600W unit for a fairly reasonable price. 600W Ultra X2 units have popped up for free after rebate, so keep an eye out for that.
Right now, Vista is not really encouraged. Gaming performance is slower than with Windows XP SP2, although with patches and driver updates, this should hopefully change. If you're concerned with DX10, don't expect games to require it for awhile. It's a new OS, and driver performance still leaves much to be desired.
Well, given that Core 2 Duo is the best bang for the money, looking at an LGA775 motherboard for starters is ideal. Unless you're looking to run a pair of video cards in SLI or Crossfire, a motherboard sporting the P965 chipset is a good starting point. Something like the MSI P965 Neo-F is around $80-90 on line. I picked one up recently, and it's a nice board. You may also consider an ECS P965 one, as they've shaped up over the years.