It depends. Let me share my thoughts in a same dilemma:
If you work 90% in ProRes, your iMac should suffice. Bottleneck here could be the disc speed (hence the BlackMagic Disc Speed Test app, you should use it). An SSD, though hypothetically able to run at 500-600 MB/s, is hampered by old SATA-controllers of your motherboard (I don't know about your Mac, check in system report) or by external connections. No USB 2.0. Best are PCIe or Thunderbolt. Raids. Assigning cards for read/write-only, not using the system drive for footage in the first place.
For FCP X, disc speeds set aside, more RAM and more graphic power make it faster, but it should run fast enough on your machine.
If you use Resolve (lite), then you best have two Nvidia cards, one 4 GB for computing (I know they say 2GB were enough, but I read otherwise), a slower one for GUI (lite can't use two cards in the way FCP X can, you'd had to upgrade to the full version, refer to the system requirements on the BM site, Resolve is touchy with cards and drivers, openGL does, for example, not include Radeon cards which are supported by Apple). Probably Resolve would be the reason to give up Apple, since the costs for upgrading an old machine (if possible at all) easily exceed those for a state-of-the-art PC. I can't tell if the Photon Envy would work, but 16 GB RAM max? ""Photon Envy" wears an intimidating look. Adorned with Green LED fans and lighting, your friends and foes alike will be certainly green with envy!" - or glare red with schadenfreude. This thing (film quiz) is so ugly, it must be a modern masterpiece.
The third option, at the same price point, would be a Hackintosh. The $4000 new MacPro can be assembled for ~ $1500, with even faster (but internal!) connections. That's my favourite plan.
In the meantime, as others, I see raw as of little if any advantage over ProRes for most situations. More moire, more noise (noise can be avoided almost completely by ETTR, which requires enough light, as should be a matter of course for a cinema camera). The noise can be taken care of by Neat inside the NLE (after i.e. developing the raw with cliphouse), by ACR and for Resolve by the full version or by Neat's own OFX-plugin, suitable for Resolve lite (now also supports AMD graphics) - Resolve again meaning a new computer, especially with Neat, which is famous for slowing down performance, and it'd need to be the first node! The moire apparently is invited by utilizing the utmost lens' resolution and can be reduced or avoided by softer lenses or open apertures. You sacrifice sharpness for a moire-free image, old recipe in DSLR videography (you indeed increase resolution then, because the moire decreases it). This brings the sharpness down to the level of ProRes ...
Keep in mind, the new Amira in it's basic version ($ 30.000) has only ProRes (not HQ), and only in Video-LOG (rec709). Why make everything in raw? If it's good enough for Arri, it's good enough for me ;-) (of course not, how can one compare a meager broadcast camcorder with a cinema camera!)
The first thing I thought after I saw the first clips from my new Pocket was that I'd never do anything else with it but raw. To grade the film-LOG ProRes so that it looks even as good as GH2 or G6 video isn't easy. And the raw clips seemed to be so much better at once (shaky camera and bad focus ignored - the colors, the nuances!). But the more test shots you make and compare and the better you get at extracting all beauty of the flat ProRes, this view changes. There are situations for raw, but they are rare.
You want to make only some beauty shots with raw? You can, you just need to invest some time and round about $50 (for cliphouse), if you have no AAE already (which reportedly has the better debayering method compared to Resolve).
What about grading ProRes with the hobbyist tools inside 'iMovie Pro'? Actually, if you take a more scientific approach than just applying looks (I tested and detest Filmconvert), you can build a similar pipeline as in Color ('rooms') or Resolve ('nodes'). Tracking vignettes? You could add the Mocha tracker within SliceX. Comparing and conforming shots in the timeline? Easiest thing: Connect a still from one typical graded shot, shove it over the other shots and crop it accordingly to make a splitscreen. Imho FCP X can be used better for grading than the NLE Premiere, because color is not an 'effect' that has to be loaded, it's an attribute of every clip that exists in the (if you choose so) ever-open info-window.
Seriously, I believe Resolve is the best software for grading, but it also is the most expensive, at least for us. I follow the advice of a friend: Either soldier on with your slow and outmoded system (MacPro 4.1 Quad Nehalem, 16GB RAM, GT 120) for now or buy/build a new one with the best specs affordable. But don't invest in any hardware to make your old system faster! Sounds reasonable.
