Premiere and I go back pretty far, so I'm naturally feeling a little strange considering jumping ship to the Mac world and Final Cut. I'm running an editing box I built, 2.8 GIG Quad Core, 8GB RAM on Vista 64. I'm using the Creative Suite 4.

Been finishing up a film I shot using an AVCHD camera, and have noticed some pretty significant slowdown. After Effects doesn't like to play very much either, slowing down during preview render, lagging out at times. Trying to decide if A) a Mac Pro is the way to go, B) Upgrading to a new PC with better components is better, or C) identify if I'm just doing something wrong to cause bottlenecks. I'd love to stay with Adobe, as I'm entirely familiar with it. But the slowdowns and occasional crashes are becoming cumbersome. Also, I'm exporting my film (1h45min) and I'm experiencing about an 18-24 hour encode time. Is that normal? Taking over 6 hours to just load into Encoder after hitting "go!" ? I've not yet worked with a project this size so I'm not sure if my concerns are justified or not. Any comment will be helpful in understanding.

Premiere cannot be trusted with projects this large. That's a sad fact of life, learned the hard way. You will have tough time with AVCHD in FCP as well (transcoding to ProRes is really helpful), but it's much more stabl, especially on larger projects.

seems your config is a bit out of sync.
" I'm running an editing box I built, 2.8 GIG Quad Core"
2.8 quad core is slow, you would better had a 3.2 core2duo overclocked at 3.8
Ram speed here is also very important , nothing less than DDR2 at 1066.

"8GB RAM on Vista 64. I'm using the Creative Suite 4. "
CS4 is pretty slow, and i doubt Vista even on 64 will help.
Try an upgrade to win7.

"Been finishing up a film I shot using an AVCHD camera"
AVCHD is the hell, but nothing oblige you to stick to this format.
You can either convert to faster codec like cineform or use accelerator (like CUDA).
Cuda will require a high end nvidia card and a software.
this one DIVIDE FRAME: Software solutions for the broadcast industry (http://www.divideframe.com/?p=downloads) is pretty recent but seems to work.