LGA 775 is not tricky at all. I install into LGA775 sockets regularly.
And the push-pin stock Intelcoolers are not too bad, but I boxed mine up and replaced it with a Zalman. Make sure you get a big enough power supply with multiple 12v rails. Preferably modular cabling. Get rid of your old IDE drives or make them external storage and put SATA drives in the new rig. Modular power supply cabling and sata drives really cleans up the inside of a case for airflow. Speaking of airflow, find a case that takes multiple 120mm fans. Big, slower-speed fans move a lot of air and keep the noise level way down. Look for a case with the USB and multimedia jacks up high instead of down low. If your case sits on the floor, why reach all the way down to plug stuff in. Oh... and high speed, low-latency memory. It's not expensive... newegg regularly has huge rebates and I'd rather drop one step in processor speed and go bigger and faster in the memory department to get that "bang for the buck".
Find the "sweet spot" in processors. Where does that big jump in price happen? Take the one on the low side of that jump. The bleeding edge just gets you bloody.
For nVidia.... no GTS cards.... 8800GT or better, and SLI if you can afford it and your motherboard supports it.
Oh and I believe you can reactivate that Vista. Call MS and tell them your computer died and you are reinstalling on a fresh hard drive. They'll give you a new activation code, but you have to get to that point in the install where it asks you for the code and fails... a phone number will come up to activate by phone. I have a copy of Vista Ultimate sitting here I'll probably never use, but I keep it because I can use that license to install XP
Find a Mac-using-college-student who attends a university with a Microsoft Campus Agreement. Students at our campus get Vista Business for $7.95 and Ultimate for $14.95.