When it comes to running Debian, or any Linux, on a server generally you need to look at two components. Will the network adapters be supported, and will the storage controllers be supported under Linux. On servers you generally don't care about all multimedia devices (video/sound/etc) that are more of PITA to get working.
The fact that there is a RedHat choice strongly indicates that it is possible to run Linux on a given piece of hardware, but there is a chance that a particular version of Linux will not have a driver compiled or built for a given kernel. You can almost always with a little effort, backport a newer kernel, re-compile a driver.
It is quite a bit more difficult to get a storage controller working over a network card. If the distro you are looking at doesn't have support for a given storage controller, you may be looking at re-mastering the installer, and building a kernel with support from another system. If the network isn't supported out of the box, you usually just have to get the source onto the box after it is installed, then compile a kernel module.
Since Redhat is an option you can usually go look at the drivers/downloads page, and see what they offer for downloads. Do they offer the source for a custom kernel? Do they offer the source for a storage/network driver? Those may be signs that it might be a bit more difficult to get Debian running.
Dell usually offers many different choices for Storage controllers so the model number alone is not enough to get a definitive answer. Usually you need to decide which storage controller options fit your needs, then verify that they will be usable with the kernel you will be using.
One other thing I will point out is that since Debian is very idealist about software freedom they have separated a lot of firmware out of the kernel the deliver as part of the standard installer. This may mean that, while it can work, it will not work with the standard installer. You may want to just make a point of always using the installer they have that includes the non-free firmware. http://wiki.debian.org/Firmware
Another highly anecdotal point, I have more issues with Broadcom network adapters under Debian then Intel NICs, that doesn't mean a given Broadcom nic won't work for you though, I am just saying I have gotten to a point where I always choose the Intel option on the server configurator.