Had a bit of first hand experience with this recently. I've always known Dell has devious pricing systems, the same system will have different starting prices and instant discounts applied to it depending on how you get to it and result in systems with the same specs being priced hundreds of dollars apart from one another.
I own a business and needed some systems quick for new hires. We have a line of credit through dell which has come in handy a few times. The new hires were going to be working heavily in the Adobe suite and needed some firepower but nothing crazy. I just could not price a system with the specs I wanted. I called up sales and they told me that it was impossible to get the system I wanted with 64-bit Vista despite that, when both options were available, there was no price difference, the sales guy made some nonsensical reference to the motherboard (was getting a intel quad-core). I asked could I get the 2GBs of memory that came with the system on one DIMM instead of two, but this was not possible because the system wouldn't support it.
In the end I broke down and now and ordering three systems worth of parts from Newegg, which, of course, satisfies my inner geek but has lead to significant delays in getting the hardware I need.