1st Build = 100's of Questions

I recently got heavily into computers in the last 6 months and now want to build a system that can easily handle some rather strenuous tasks on my IBM Thinkpad T42. The things I want to do is convert video files like mp4 to wmv and I want to be able to get into online gaming potentially without losing during the experience. With the past research over the last few weeks I've got it narrowed down to wanting quad core, an alright video card, and being able to overclock, other than that I'm really needing suggestions as to what to look out for I just wanna be able to expand as need be with a ton of room for ram. Sorry about the long post but this is my first build and I wanna do it right!

Plan on buying over time so probably $200-300 per month starting next month, able to be solely devoted to the project.

If you can spend $200~$300 every month, just save for 4~5 months. Then, you'll have enough money to build yourself a decent machine. You can ask again what to pick once you've got the dough.