Actually, Mental Ray satellite (as craptastically buggy as it is) still had a 8-thread limit under Maya 2009 sp1a (patch notes say they removed the restriction, but watch your CPU usage with a dual Nehalem and tell me it's not locked to 8 cores still)....
But it's not so much that... I mean if you've got the budget for Renderman Pro or Mental Ray standalone, you've got the budget to build a farm properly, and yeah an i7 is most definitely worth every penny, Nehalem Xeons are great too if someone else is paying the tab. If you're buying Renderman Pro, you're likely getting Xeons.
I've got 3 identical i7s, a core 2 quad and a core 2 duo for rendering here, and whenever I'm doing hair (shave in particular) or some dynamics plug-in work, I get significantly better render time using the core 2 duo due to the nature of multithreading. If you're only using 1 thread on a hyperthreaded quad, you're only using 12% of the available processing power, and it's more efficient to use a slower processor that you can utilize more of, if that makes sense. Same goes for a few repeat offending after effects plug ins (cinelook and magic bullet come to mind here). I mean you could go a step further and run 8 single core VMs on an i7 to saturate the CPU doing a hair scene (actually works pretty well with linux VMs in a pinch).
I do love the i7s from the very bottom of my heart, though. I'm getting almost a 50% gain in frames rendered between q4400s and i7 920s using Mental Ray in most cases.
I'm a big fan of imaging my boxes, as you mentioned. Particularly with a small shop it can be an absolutely maddening time sink to troubleshoot faulty nodes.
I can't think of the last time I ran into the video card issue... might have been lightwave way back when, but I've seen it. The real point here is "make sure the stuff you're buying is suitable for the work you're doing". If the bulk of your software is single threaded still, an i7 box really might not be the best choice. For most folks doing this professionally, it's an awesome choice though. If you're editing HD or 2k/4k over a network, you need to spend a little extra cash to make sure your disk reads and net throughput are up to snuff.
With that goes: if you're using enormous float textures and displacement maps out the ass, you're going to lose a substantial amount of time on disk & network throughput. Go gigabit ethernet at a minimum (it's cheap) and get a nice, fast raid 5 or 6 for your primary storage (and get another big disk to back it up with, at a minimum). Just because you don't need much storage on those render doesn't mean you can cheap out on the drives (ie 5400 rpm throwaways). Disk and network throughput matter and the matter more as you add more render slaves.
Regarding Vista: it depends on your hardware. Up through SP1, I still had a couple of mainboards with unstable (*cough*nforce*cough) drivers in Vista 64, as well as a few pieces of software that required UAC off (eww). Gave up on Vista then and I've had really good, rock solid stability in XP64. Win 7 seems to be shaping up nicely on my 2 oldish Athlon x2 test sandboxes. I think this really depends on preference and available hardware, personally, but it's going to be a non-issue come October anyhow (and thankfully).