Good point. I have an interactive marketing business. Most of my work is done within CS6 (PS and DW) and I make heavy use of VMWare running Win7.
If you are running a 2008 2.8GHz 8 core model then you are pretty far back in hardware virtualization support. Here is a chart that Intel presented for Haswell
http://www.anandtech.com/show/6355/intels-haswell-architecture/11
5 back from Haswell is a yellowish, substantially taller, bar which is probably indicative of your machine . The Fry's special is the 3 back red bar. The Mac Pro 2013 is the light blue before Haswell.
Those are basically indicators of the overhead of running virtualization. An active VM itself is going to consume far more resources, but is indicative there is newer hardware support just don't have in either your 2008 or effectively de facto 2010 technology foundation in that Fry's special.
RAM probably is one of the primary root cause issues while running RAM hungry apps in each of the OS instances. If Win7 is just for reference browsers to check work then can try throttling that virtural machines resource usage a bit as short term work around.
It isn't just how much RAM using but where you are at in terms of Page outs / Page ins. If churning on that heavily then basically need more RAM ( although I guess OS X 10.9 should help with that also ).
The Mac Pro 2013 6 core config is probably substantially faster than your 8 core model. If that is a sub 2.6GHz 12 core the 6 core model is pretty close. There is a pretty hefty jump to get up to 8 cores and head room over just about any old 12 core model.
You seem to have a couple of laptops. That may be a contributing factor. If going to share devices between several Macs, Thunderbolt may work better.
If you go the MP2013 route you may want to BTO the entry config. Most likely you'll want another set of DIMMs in there to push substantially past 16GB. The entry's 12GB are easier/cheaper to replace.
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2 Cores and 2GB of RAM. I've played around with those settings and that seems to be the sweet spot if there is such a thing

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Yep. I keep an eye on my Page Ins and Outs and there isn't an issue there.
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The interesting thing is that I don't remember this being that much of an issue with WinXP which is in the same VM folder on the same spindle.
The page in/out of the guest OS also is something to keep an eye one. Although if move its virtual drive storage to another storage device that would decouple impact on the host OS X and its apps. XP's smaller footprint may have fit better into the limited box you were putting it in.