OK, a few questions (not disagreement as much as desire to understand ^_^)...

1) Why a different dual-core processor rather than a Quad-core one? Would there be possible issue fitting it on my mobo?

2) How big of a power supply should I be aiming for? I'm still using the 300w it came with, but I know I need to replace it once I start upgrading stuff. Do you think the 550w you linked should be good or would it be more beneficial to grab a 700w or something? Also, on the power supplies, how do I go about making sure I end up with one that's going to fit my case? I made the mistake a few years ago of trusting a guy at best buy after mine melted and ended up with a power supply balanced on the top of my case - ran nice and cool though!

3) As for ventilation, other than the new RAM, I'm running everything as it came from the factory - is a new cooling fan something I should be thinking about getting?