I'm not sure what you mean by "Commercial". There's no hardware difference between hardware which Dell sells you, and hardware which you put together yourself. If you're curious what the maxium amount of ram a vendor will sell you, that various from company to company. The most ram offered I've seen offered is 32GB of ram on Apple's Mac Pro with two quad-core processors.
As for theoretical limits, it varies greatly based on hardware, and if you've got a 32bit or 64bit operating system. On 32bit operating systems, there's a limitation which makes the operating system, and the "Memory Mapped I/O" module on the computer share the same pool of RAM. This limits the RAM your computer can use to 3.2GB of ram. Even if you put 4 sticks into your computer, it will only be able to address 3.2GB of ram.
If you've got a 64bit operating system, then it theoretically can support up to 128GB of ram. The only exception is Windows Vista Home Premium then it has an artificial limit of 16GB. However, although your OS can accept that much ram, the hardware now limits how much your computer can use. Older Core 2 Duo machines are still stuck at 4GB of ram, with the exception of those using the new Nvidia chipset motherboards, which support 8GB of ram. Core 2 Quads can support up to 16GB of ram, which means that a few high end machines with two Core 2 Quads (such as the Mac Pro) can use 32GB of ram. The motherboard on the new quad core Intel i7 processors currently accept 24GB of ram, but to get up to that amount you'll need to use 8GB ram sticks... which aren't readily available yet.
I have a feeling that my answer's a bit overkill, but hopefully you'll find it useful.