The reason you can't come up with a good configuration is Apple doesn't make one. If you want to switch you're going to have to take a hit somewhere.
I'll jump in with the contrary view. Even the quad-core Mac Pro is way overkill for what you are doing. It's a fantastic machine, but it's also a truckload of money. It uses server grade RAM so even with 3rd party sticks it's still going to cost more to bump than an iMac or Mini. While it is upgradeable you can't just go grab any video card and shove it in there. You've got to use what ever Apple is willing to code drives for. While internal drives are nice you can use external drives or NAS to bump the storage of an iMac or mini.
The current mini would actually work for you albeit just barely. It's due for a refresh that will hopefully include better on-board graphics if not the dedicated video card many of us want. I'd wait a few weeks for an updated mini then go take a look at one. Replacing the mini every 3 years will cost you less than replacing the MP every 6.
If you have to buy now I'd consider the 2.4 ghZ iMac. The 20" has a really bad screen for image work, but you could use your current monitor as a second and just do all of your image editing on that, or you could spring for the 24" which has an OK but not great screen. Glossy is your only option with the iMac so you'll have to decide how you feel about that. It's also worth noting that as soon as someone makes a 4GB stick the current iMacs can run 8GB of RAM.
