The only thing there that benefits "significantly" past 4 cores is video encoding. That is the one area where an 8 core might be faster. In terms of overall speed, you'd be significantly better off with a 2009 quad than a 2008 8 core considering what you use. As I mentioned I don't really like imacs, but I think the current mac pro at current pricing levels is kind of expensive, and the one you were considering isn't at all worth it. Whenever someone I know asks me what to buy these days I'm never really sure what to suggest, as they all have something significant that I dislike. Stuff like photoshop is really quite easy on modern hardware. It relies on ram more than cpu power. Things like liquify, puppet warp, etc. are being rewritten to take advantage of OpenCL which isn't so much cpu dependent either. Things I really like about the mac pros is that they're likely the most reliable overall when comparing new machine to new machine. They have much better options for addon hardware via PCI additions and internal drive bays. They're extremely quiet. I don't necessarily think that they're the only way to go in a new machine today. If you note the machine in ashman's signature, if you had that today, I'd say stick with what you have. I just wouldn't buy that from someone else at this point.
It still costs more after you buy a used computer. How much do you really want to sink into a 4 year old machine that could be phased out in a year or two? The 5870 had issues under certain OS revisions too. I remember it wasn't stable under Lion in that machine on the early builds. Either way it's $400 or so from Apple on an older second hand machine, and when it comes to determining what does or doesn't make it into the next OS revision, they calculate on hardware of that era. I remember those cards wouldn't support OpenCL. It came up on this board with one of the FCPX updates. Anyway the 4,1 isn't a bad idea, as you can make it appear to be a 5,1 meaning that it should be possible to use it on the current OS much longer. The cheaper memory is also a big advantage as you mentioned.