I decided against the iMac route. They're really nice looking, but I couldn't get past the locked down hardware thing, and the prospect of having to make compromises elsewhere (some of which defeat the clutter-free looks of an all in one device).
So, I had a PC built for me. Chose the components, ended up with a great machine for less than half the price. Actually got this on Monday, but had a bit of trouble with getting some hardware (FW card) and getting my MOTU to work with it (had to slum it for a few days and use my AFXII as the sound device - ugh. Never buying MOTU again whilst I'm on Windows).
ROG Ranger VII Mobo
i7 4970K 3.6-4ghz (leaving it at 3.6 for now)
16gb 2400mhz RAM
MSI Nvidia 750ti 2gb
500gb SATA SSD
3 x 2tb HDD
Bunch of quiet cooling options on fans and PSU
Aerocool DS 200 case (looks pretty cool):
I grabbed the Dell 27inch Ultra display to go with it, too.
Finally got all my shit installed on it today. It's working like a charm. Boots from cold to working desktop in like 10 seconds. Everything seems to be running really nicely, too.
I was working on some soundtrack-style music a couple of weeks ago on my old machine, and the CPU was hitting 90%. The same project on this machine, with some extra VSTs, and it's just hitting 40%. Gonna try working on some more orchestral music over the weekend, see if it handles what I try this time.
That color is ridiculously awesome.