Cool, this is a good idea, Dave. We've seen a lot of discussion of late about SDS in recent threads.
There is a key difference between Dell D620/820's and D630/830's, even though both boxes use 64-bit CPU's and are nearly identical otherwise. That key difference is the Southbridge. The D620/820's use the Intel 845 chipset, whose Southbridge chip (there's a Northbridge and a Southbridge) has only 32 lines to address memory. So, while the CPU can address multiple terabytes of DRAM, the Southbridge doesn't have enough lines to keep up with the CPU. Those 32 address lines limit you to only 4GB DRAM total memory address space. Again, this ain't a CPU limitation; it's a motherboard chipset limitation.
Of that 4GB of address space, you have the various parts of the computer, largely the PCI bus, that need addressing. These other parts, in Dell D600/800-series Latitudes, get about 0.75 GB of address space. That leaves you with 3.25 GB of actual useable DRAM. Note that this is address space; you're not actually allocating DRAM (it's because of how computer addressing works--long story). That means if you have 3GB DRAM in your computer, you will see all 3GB. However, once you install anything past 3.25GB (e. g. you install 4GB), that DRAM past 3.25 GB will not be seen because the PCI bus is using those particular address lines.
This is why it doesn't make economic sense to install anything past 3GB in a Dell D620/820.
Side note: this ticked enough owners off and became such a well-known problem that Intel got Microsoft to issue a patch for Windows XP so that the OS would no longer report 3.25 GB DRAM (which Windows *actually* saw), but rather lie and tell you 4GB! The trick MS used was to read what the Intel BIOS reported was physically installed instead of what DRAM was actually available like you're supposed to do. Gotta love "Wintel", eh?
GNU/Linux and the BSD's, of course, continued to tell you the truth.
The situation changes with D630/830's, which use the improved Intel 865 chipset. This was the major change between D620/820 and D630/830; everything else is basically the same. Fully aware that people were getting ticked about their "4GB DRAM" not being fully used, Intel modified the 845 chipset and added a 33rd memory address line. This new chipset, they called the 865. With that 33rd memory address line, now you have 2^33 bytes, or 8GB, of possible address space. So, with the PCI bus's "tax" of 0.75 GB, you get a real-world capacity of 7.25 GB of DRAM. So, in these, it made more economical sense to fill the box to DRAM capacity.
Now, for purposes of SDS, does this matter?
For Windows XP-based systems, not really. I use D830's for SDS, both for the larger screen and the better Intel 865 chipset. I have 3GB in one computer and 4GB in the other. Both are running Windows XP-based images at present. There really isn't that much difference. However, I may go to a Windows 7-based system later on, in which case, I'll be slappin' 8GB in the laptop, because anything from Windows Vista on is a pig when it comes to DRAM.
So, which one should you get? Price being equal or close, get the D630/830, depending on how big a screen you prefer. If you see a D620/820 for substantially less, though, it'll be fine for Windows XP-based SDS, though Windows 7-based systems will not be too happy with such a low amount. If you don't connect to the Internet, there's no real reason to go Windows 7, and I *NEVER, EVER* allow my SDS laptops to touch the Internet for several very good reasons, so Windows XP images are fine in such cases.