Best performance for $$ is the Core 2 Duo E8400. When I priced it (3 wks ago), I think it was like $170-$180.
If you do not like this chip (color, price, smell, whatever ...), when you buy chip look for size of internal cache - E8400 has 6MB I believe. The bigger the better. The speed of chip (MHz) is somewhat important, but can be misleading. If you have the $$, you can also go with "Extreme" or "Quad".
If you are going to use Photoshop or Lightroom, I recommend going with Vista 64-bit so you can use more then 3MB RAM. I had my new system specced for 8MB - I went with 1333 ones, two stick of 4MB.
Decent motherboards will run you from $150-$250 ... go above this and you are in luxury gaming type models. I specced mine with an ASrock that was in $150 range. It is SLI (2 video cards) - I had8800 GTs specced - I went with a "more generic brand" for around $140 each - forgot the brand, it was "forgetable".
Add in two DVD writers, a case, two 750GB HDs (Western Digital) and I think my total came out to a little under $800 - no tax or shipping included. I already have Vista 64-bit so that's not included.
Have not pulled trigger because trying to decide whether to go ASrock m/b or an ASUS around $250 
Primary reason I am looking to upgrade is "DPP" - that darn RAW conversion process taking too long on my first generation Core 2 Duo. Also, Elements feel "slow" - if it's anything like Photoshop, extra memory should make it happy. AND I heard Lightroom 2.0 is optimized for 64-bit computing - YEAH!! Probably can't afford LR2 after computer, but can always wait for it be available academically.