My machine has 16GB total. I give XP 512mb and 1 processor/core. Ubuntu/CentOS 2GB with 2 processors and 2 cores, OSX has 4GB also with 2 processors and 2 cores per processor. I actually have 2 XPs, 1 with IE6 and another with IE7. I've noticed a bug or two that IE9's developer tools didn't mimic exactly, that or it was a combination of XP+IE6/7. Either way we had a bug once that couldn't be reproduced in IE developer tools but we could see it in BrowserStack.

I've not tried a beefy 8 core OSX to try Final Cut Pro on yet, that would be interesting to see performance output.

All VMs run in bridge mode, connected directly to network so they have their own static IPs, makes it nice if I want to connect to a database on a VM

I also bought a separate SSD to run all these VMs off of

Note, that if you prefer OSX to Windows from a UX perspective, you can do the same thing with VMware and run Win7 in a VM. Really we all want linux for coding but a better desktop of our personal choice without having to read a ton of docs and deal with random bugs/gotchas for specific this or that's.

Not the parent, but I work the same way, and I tend to allocate 2GB RAM per VM. I haven't really used my Windows 7 guest yet, so I don't know if that one needs more, but for everything else it's enough.