PC:
Customization.
Unrestricted Upgrading.
Alot cheaper.
You don't pay for a Linux Distro as an OS.
You can play many many more games.
Wider support for various file formats.
Mac:
Overpriced.
Underpowered.
Look okay, i guess.
Apple often try to market old tech, for example, check...
Please present proof that x86 is more stable than x86-64 in terms of windows 7...
There hasn't been issues with x86-64 since the early stages of Vista...
Although Ubuntu Linux is great, it doesn't offer the usability of Windows based OS's, sadly.
A 140mm fan with a 120mm heat sink?
You cn't get much better than Noctuas in terms of performance, not to mention they're whisper quiet.
@OP, are you planning on overclocking?
Also, i recommend more than a 700watt PSU for that rig...
Go with 1000 watt for future proofing, thats what i did and i...
You can't use an AM3 CPU with an AM2 socket...
You need at least AM2+.
Just buy a new motherboard, you can get one for ~$120 on pc case gear.
edit: Sorry, AM3 CPU's will only fit into an AM3 Socket...