I did a lot of research a couple of months ago on color management. You can spend a lot of time in this area as it gets much more complicated than it would seem on the surface if you really want to understand it deeply.
A big consideration is what devices you want to color manage. If you only want to color manage the monitor(s) the less expensive devices will work. If you want to manage your printer as well - then you need a spectrophotometer, which is more expensive.
Here is a good blog post to read on this from Ron Martinsen http://www.ronmartblog.com/search?q=colormunki
He recommends the ColorMunki.
Re: Graphics cards - everything I have read says that Photoshop and lightroom are CPU intensive, not GPU intensive. I built a PC this summer with the latest desktop i7 processor and just used the on-chip GPU and my system is very snappy. I have a SSD as the main drive and 16G of memory. I don't even have a dedicated graphics card. I have 7D size raw files and a fairly large number of pictures (10,000+) and it does fine.
Re: Dual monitors. I have two Dell monitors and the setup works great. Make sure your system board and/or graphics card(s) support the proper connections for your monitors (VGA, Displayport, DVI, etc.). In my case I have one monitor connected to VGA and the other to DVI - not optimal but I wanted to use the monitors I had and avoid buying a graphics card. (Note the max resolution differs between an analog connection and a digital connection on the same monitor). The color gammut of my two monitors is not the same, so the colors don't exactly match. (I suggest getting identical monitors if you can) Also, most consumer (low cost) monitors are designed for landscape viewing, and if you turn the monitor to portrait orientation, the angle of view is very narrow. I have to be looking straight at the monitor to avoid distortion - looking at even a 30 degree angle isn't good.
I use the second monitor in portrait mode in lightroom when previewing portrait shots - it works great. Lightroom does a very good job with it's dual monitor support.