That's actually not a bad system for a Dell. If the specs linked is the actual system, then you do indeed have an open PCI-Expressx16 slot, so you have a wide variety of 3d cards to choose from.
I do have a little bit of bad news though- while the X300SE is a really, really low-end 3d card that most certainly needs upgrading... Flight Simulator-X really is a slow, stuttery mess on even the most upper-end systems I've been trying it on.
You'll get definate improvements with an upgrade, but it'll still be a chore of configuring and re-configuring to get decent/acceptable performance from it.
I'd truly recommend an X1800/X1900 or 7800/7900 series PCI-Express card for FS-X. While you'll still be limited to very, very low framerates- you really don't need super framerates for a flight sim. But going with a modern SM3.0 card will improve smoothness and framerate consistency (to the extent of your CPU, which will also be very taxed by FS-X). You'll want the smoothness so you can actually land a plane without belly-planting on the runway from stutter lol.