I agree with Doug's suggestion that mid-range Quadro's are fine for ProE (if your budget allows), and relatively future-proof for non-ProE applications. I also agree that the high-end cards are a complete waste of money! As you probably already know, the most important factor in ProE workstation performance is cpu single-thread horsepower.
Contrary to Intel's (and virtually all the OEM's) hype, Quad-core cpu is a total waste for ProEngineer (now and thru Wildfire 6) as Part regeneration is not multi-threaded (and won't be through at least Wildfire6). The current ProEngineer cpu champ is the Intel 670 (dual-core Nehalem @ 3.46 ghz, 3.72 ghz in Turbo mode). The only first-tier OEM workstation products that offer this cpu right now (as far as I know) are the Dell T1500 and the HP z200. The funny thing is that both Dell and HP market these as "low-end" systems.
It has always been my experience that on a single monitor, with the highest display resolution settings, that all Quadro graphics cards run ProEngineer about the same (see example below). The 5xx series Quadro has usually been my recommendation, and I have never witnessed a ProEngineer problem whose root cause was the 5xx series card.
I have discussed this issue directly with tech folks inside PTC, Nvidia, and HP, and they are aware of the lack of ProE performance delta across their entire graphics product line. I have seen no data nor heard any claim that the lack of performance delta changes with Wildfire 4 or 5.
Dual display with brutal processes running on both screens at the same time might be a different story, but I don't know any ProE users that actually do that. We have plenty of folks with dual monitors (and 5xx cards) that are doing just fine... but they are only doing graphics-intensive (> 5 million polygons) stuff on one monitor at a time.
I have been running Radar and Antenna Top Assemblies on a HP z200 with a Quadro FX 380 for the last few months with no ill effect.