You know, it really just depends on what kind of workloads you're going to place on this. If there are alot of complex database operations it would make sense to going with a RAID 5 array for the Hard Drives to increase speed. Realistically though this might be a bit of overkill. I'm assuming that your VMs will just be basic LAMP stack servers with your custom code loaded on them and will not have to rely on other VMs or outside servers. If you want to setup something more complicated (like Eidos Methode which includes a cacheing) then it gets a little more complicated .
I recommend going with 8GB. You run out of RAM fast when you're testing different DB techs and development tools ^_^. If you're going to use more than one VM at a time, then you may want to step it up to 16GB just so that workstation will have memory for some time to come. If you're going to run code with alot of disk access, I recommend a RAID 5 array, but if not then your 2TB drive is fine. You may want to mirror it anyway just in case.
What area (how hot does it get)
Also, I know you didn't list reliability on your consideration list, but I feel obligated to at least suggest that using non-server parts does have some drawbacks in that area.