I use Newegg for PC upgrades. They have several video cards that are listed as compatible with Windows 7 and have enough muster to run HD video in WMC. I recommend nVidia Video Cards
As for RAM, Kingston
has a tool to use when upgrading memory. Make sure you are very specific when using it. Once you have the Kingston part number you can search for deals online or purchse directly from Kingston. Most retailers will not take RAM back if you bought the wrong one, or even if it is defective, so I deal with the manufacturer(s) directly. Make sure you purchase a matched set.
Again, if you visit the Dell Support website for home users, you can find Windows 7 information
and specific recommendations for your exact PC by starting with your Dell Service Tag
, located on a small tag on your PC case. I do not purchase upgrade or replacement parts from Dell diectly as their warranty terms are generally less than if buying direct from the Manufacturer, 90 days vs. 1-3 years, typically. Grab the specs from Dell and buy direct.
Before proceeding to upgrade a PC's hardware, make sure you use safety, always power down and unplug everything first, etc..