First Build

Here is my currrent system (All parts are 5 years old):
Processor  - AMD Athlon 850 Mhz
Memory     - 768MBs DIMM SDRAM PC133
Sound Card - SoundBlaster Live! 5.1 Platinum
Video Card - ATi Radeon 7200
Speakers   - Altec Lansing 5.1 Set
Monitor    - 17" KDS Avitron Flat-CRT
I use this computer primarily to play games (The last "FPS" game that I've played was one I recieved in a cereal box called Chex Quest), surf the net, P2P, emulation (I know I should just buy a console), watch movies, burn DVDs, and play music.

I am planning my first build on my own in the five years since I got this one. Consequently, the age is showing a little.
I have been playing catch-up with the market nowadays since the middle of August, and getting a straight answer is like asking to open a Baskin-Robbins© branch at the depths of heck.
(I have no preference for companies AMD/Intel, Nvidia/ATi, etc.)
So I have a few, albeit hazy, questions about today's components.

1)Are Dual-core CPUs worth buying right now for performance
   enhancing reasons? (Guh, what subjective words!)
   If so, which would give the calibur of an Intel Prescott
   3.4Ghz or higher (I am attempting, if in vain, to limit the
   price to $3XX)
2)If dual-core or single AMDs seem better, does the fact that
   Intels use DDR2 RAM as opposed to AMD's DDR1 support only
   make any difference edge-wise?
3)Is there any noticable difference between SATA1 or SATA2 HDs
   as per performance? If you're wondering, the chipsets for the
   Intels (except $XXX chips) will not support SATA2 drives,
   while AMDs won't support DDR2 memory. (I would be happy if
   they did however)

I really need to have this project done soon, because my income will be cut in half for eroneous reasons come springtime.
Therefore, waiting isn't much of an option..
  Geez, I'm sorry for such a long post.
                Thank you for your time, knowledge, and support.

Recommendation: buy a Dell. Buying from parts is not as good a deal as it was 5 years ago.