Don't bother getting the Zalman 9700. It is way more than you need to spend to get to 3GHz. The Arctic Cooling Freezer 7 PRO is the best price to performance ratio cooler available ATM: http://www.newegg.com/Product/Produc...82E16835186134
I think the best option would be simply to buy an aftermarket cooler as well. The reason being that your motherboard probably doesn't support the C2Ds. As for upgrading to Vista. I seriously do not recommend it for what you're doing with your PC. Firstly because Vista requires A LOT more resources than XP. So you might find that your PC is a bit sluggish in Vista, and secondly because it won't bring you any advantages over XP. You're not a gamer so you don't need the DX10 support. Stick with XP until your next upgrade.