Well, here's my $0.02 (USD)
Like Stephen said, four cores is plenty. Higher the speed, the better. That changes with FEA and the other programs you are running.
ECC is probably good to have if you keep a very large assembly in memory all week long or else frequently do heavy and critical analyses. But then again, memory corruption is rare and you may never even notice it if it did happen. But reliability is peace of mind....then again, high speed Xeon's to get ECC aren't cheap. So it depends on your budget. I don't think you'll notice much difference in the speed of RAM unless you are comparing leaps in generations of RAM.
Over the years for non-professional cards I've preferred Nvidia's cards over AMD's due to driver quality. But I've used many Quadro's and one Firepro and their drivers are very reliable. Quadros always seemed to have a small edge in speed over Firepros. Creo 2.0 build M090 and above support better transparency for Quadros but earlier builds for Firepro's. I don't think you could go wrong with either. But if I were to make a decision with no mind to budget, I'd go with a Quadro. You can probably avoid the very high end (5000 level quadro) cards. I've run sessions with 31k parts on 2000 and 4000 level cards just fine.
I have 32GB in my machine since I have large assemblies and do FEA quite often. I'm even contemplating running the software or my "scratch" space for analyses in a RAM Disk. Almost daily I hit about 15GB without doing any FEA. There are higher quality settings for graphics in Creo but it will eat your RAM up quickly depending on how many parts you have loaded into session and what graphics quality setting you have.
SSD's are nice, no brainer there.
If you are pulling common data (formats, colors, part files) etc. from the network, a high speed and reliable network is imperitive. If you don't have a high speed network connection to where this data is stored, it may be wise to replicate the data locally and synchronise as needed since a slow connection can cause Creo to pause and it can be frustrating.
Input options. I usually have my own personal mouse and 3D mouse that I take to work and customize, but that's me and some people think I'm odd. 
Monitor choice. More pixels, the better. I wouldn't go with anything smaller than 20" and be careful with wide screen monitors....menus, etc. they limit vertical space....extra space horizontally becomes useless. I've used 30" monitors, they are nice, but I'd still prefer two monitors so I'm not constantly resizing Creo windows and it's easier to compare things side to side.
Speaking of two monitors if I had a choice between ECC RAM vs. Two monitors, or a professional card vs. two monitors, I'd go with two monitors. Make sure they are the same size and model number with color adjustments. You'll go crazy if one monitor is slightly off from the other or you have to move your mouse to a certain location to switch monitors due to a difference in size. There is a slight performance hit by going to dual monitors but it's barely noticeable and the advantage of two monitors seriously outweighs any drawbacks.