To Consider
Danny -
You seem to have very specific needs and lots of good advice here if* you end up dedicating your system for FC.
To consider:
CPU - The diff between the two speeds of the CPU can make a difference based on the types of applications you run. Applications that misbehave will try to take all of your CPU time and you end up in a waiting game. Faster CPU will get over these pauses faster (assuming it hasn't freezed you out entirely). In general work, items such as converting large files will be a little bit faster but not significant. FC does take advantage of processor speed but in your case, the diff between the two is relatively small with no major gain for the price.
Scratch Drive - an absolute must for what you are doing. While I am usually not one to overly recommend "solutions," you would really get a benefit from using Firewire 800 to an external drive. To speed up that external drive, consider using a striped enclosure. This is an enclosure with 2 drives that work together as one and over 50-80 percent more speed than the single drive. The drawback to this mode is that if one drive fails, the entire volume of data is lost. Just make sure you are not saving anything to these drives but only use it for scratch and, you do back ups.
Back Up - another absolute must. Plenty was written on this and you can pick how you want to do this whether its an external drive, external mirror drive, NAS or.... etc.
Video - in spite of what people say about the video, opt for a better video card. FC may not NOW take advantage of the GPU, but could in the future and certainly, you will find that you may add other applications that will. Why live with regrets over a couple of pounds well spent? Remember, some playback software do use the advantage of better video cards. For me, I have high end video in my system and as of recent, one of the applications I use daily is upgraded and a new feature is exploiting the GPU/video OpenGL for faster rendering. So my investment in the better video paid off faster than expected.
RAM - you are limited to 4 gigs. This is unfortunate but livable. Mac Pro as mentioned is a better graphic station not only by CPU (quad/octo) but ability to add more than 4 gigs RAM. Nothing to do here as you know the limit of populating your iMac to 4 gigs. (Video too has better choices)
----- List
2.8 imac with min RAM offered, best video you can afford
3rd party RAM to populate for 4 gigs (easy to do, far cheaper than Apple)
External drive enclosure with USB2/FW400/800 or 2 drive stripe raid FW800.
Some type of back up solution - external drive, NAS etc.
Remember, you will want at least 2 years of use out of your system. Hardware keeps changing and we always want to get "more" <grin> so its to your advantage to consider the next 1-2 years on the machine you decide to purchase.
- Phrehdd