It's hard to make a direct comparison. Your X300 is at least 4-5 years old if not older and it was the bottom of the line in the "X" series.
I just took another look at those two cards above again. In another section under the recommendations they recommend that you have at least af 400W power supply so the 5000 series is a bit much for your system.
I just learned that we have one other consderation to make. The older system like yours probably has what is called a "generation 1.0" PCIe slot (if it has that type of slot at all. Many of the lastest video cards in the ATI line may require a "generation 2.0" PCIe slot.
You should talk to Dell first and find out for sure what type of video slot you have.
Then you have to talk with a knowledgable salesperson that can determine what cards will work on a system with your type of PCIe slot and power supply.
On the ATI site, for a PCIe 1.0 system, they have the X series cards and the Radeon HD 2400 Pro card listed. The problem is that I don't believe that the X series card are still available, and it may be the same for the HD 2400 Pro. A 2400 would probably be a best cast upgrade if it didn't require too big of a power supply and one can be found.
I'd talk it over with a AMD/ATI tech person, but you need that info from Dell....slot type (PCIe slot type? PCe 1x, PCIe 16x and and if it is generation 1.0, 1.1 or 2.0? Power supply (to confirm) wattage? Be sure to tell them that it has to have a onboard fan!!!!
A quick look at Newegg.com, recommended by one of the TM staffers, turned up a VistionTek HD 2600 Pro
. The info says a 300W power supply is required in the system requirement info.