Hey there, welcome to HTPC gaming!
First off, what are your goals for the PC? Gaming on a tv, projector, media center use, movies, etc?
A few suggestions:
1) I'd go with a blu-ray player/burner. Especially if you want to watch movies and the like, theyre pretty cheap these days.
2) Artic silvers good stuff, I put it on every machine. My machine is running on about 2 years now, super cool still, still havent changed thermal compound.
3) Look into liquid cooling, they are becoming very reasonable these days. Look into some reviews on newegg to help you decide. If youre going for a fan, i've had just about every brand and currently Zalman is my fave brand. I have this one in my pc now and love it, minus cleaning dust off every so often...
4) Memorys cheap, go for 16gb if you can afford it, I do like G.skill as well and believe they have a lifetime warranty?
5) For me, I would change case, but thats just me. I do like the cooler master cases:
The HAF line is one of my favorites to build in. Plenty of room, room for expansion if you want, great air movement, super sturdy, etc.
6) Upgrade card if you can, maybe to 2GB or run SLI/Crossfire. Again, all depends what youre using it for? I still have a ****** ATI 5770 1GB and runs everything pretty much great except at max settings. Im a fan of ATI`'s but thats just me! Sign up for newegg promos, they run specials on nvidia cards pretty much everyday, maybe you can find a deal there.
Hope this help, look forward to your reply...