The iMac screen is apparrently 8 bit, and 72%Adobe RGB, which isn't really what I wanted to hear! I have been researching others monitors, and I am leaning towards a PC with a NEC 2690WUXi2. It has a wide Gamut, around 100% Adobe RGB, 12 bit LUT, and with spectraview 2 it is capable of hardware calibration.
I would always choose dual monitors, so two 24" (or the NEC + a 24") monitors may resolve some of your hesitation over a smaller monitor? The second monitor doesn't have to be world class - it is for Photoshop Layers, Palettes, etc. In Lightroom it is used more directly to edit images for masks, etc though.
For cost comparison purposes, you may also want to start with a more basic - but still decent - monitor as a lower level price comparsion. You are comparing a lower end monitor, equivalent in specs to a $350 27" monitor, to a higher end monitor (which is probably a decent choice, just want to get you closer on $$.) There are also mid-point Dell, HP, and other 24" monitors that have near 100% RGB, etc. in the $500 range. I haven't checked the larger ones more recently, I need to avoid temptation for a while! (I bought 5 boxes last summer.)
A Win 7, 64 bit, 6GB DDR3 RAM, quad i7 920 with a 640GB or 1 TB HDD should run around $600 or less. I bought two good quality machines with a lot of expansion space for less than that 8 months ago. Get 6 slots for RAM so that you can go to 12GB using cheaper 2GB chips.
I have PCs and Macs, I use an iPhone many hours a day. Honestly I don't find much difference, especially with 64 bit Win 7, which I really like. Mostly comes down to quibbles for me, a pro here, a con there. And cost, which you said you are sensitive to.
Cheers! Have fun!